Senza titolo


474

*Received September 2001; revised March 2002.

†Send requests for reprints to the author, Philosophy Department and Center for As-
trobiology, 169 Hellems, Campus Box 232, University of Colorado, Boulder, CO 80309;
e-mail: cleland@spot.colorado.edu.

‡I am indebted to Alan Franklin, Marc Lang, and Stephen Leeds for comments and
helpful suggestions on an earlier draft of this paper. This research was supported in
part by a grant from the National Aeronautics and Space Administration to the Uni-
versity of Colorado’s Astrobiology Institute.

Philosophy of Science, 69 (September 2002) pp. 474–496. 0031-8248/2002/6903-0000$10.00
Copyright 2002 by the Philosophy of Science Association. All rights reserved.

Methodological and Epistemic
Differences between Historical Science

and Experimental Science*

Carol E. Cleland†‡
University of Colorado, Boulder

Experimental research is commonly held up as the paradigm of “good” science. Al-
though experiment plays many roles in science, its classical role is testing hypotheses
in controlled laboratory settings. Historical science (which includes work in geology,
biology, and astronomy, as well as paleontology and archaeology) is sometimes held
to be inferior on the grounds that its hypothesis cannot be tested by controlled labo-
ratory experiments. Using contemporary examples from diverse scientific disciplines,
this paper explores differences in practice between historical and experimental research
vis-à-vis the testing of hypotheses. It rejects the claim that historical research is episte-
mically inferior. For as I argue, scientists engage in two very different patterns of evi-
dential reasoning and, although there is overlap, one pattern predominates in historical
research and the other pattern predominates in classical experimental research. I show
that these different patterns of reasoning are grounded in an objective and remarkably
pervasive time asymmetry of nature.

1. Introduction. Experimental research is commonly held up as the para-
digm of successful (a.k.a. good) science. The role classically attributed to
experiment is that of testing hypotheses in controlled laboratory settings.
Not all scientific hypotheses can be tested in this manner, however. His-
torical hypotheses about the remote past provide good examples. Al-



!"#$% &. !%&%"'(475

1. See, for instance, “A Scientific Dissent from Darwinism” (2001, 23), a statement that
recently appeared in the New York Review of Books and was signed by approximately
100 (mostly physical) scientists; they listed their fields after their names.

though fields such as paleontology and archaeology provide the familiar
examples, historical hypotheses are also common in geology, biology,
planetary science, astronomy, and astrophysics. The focus of historical
research is on explaining existing natural phenomena in terms of long past
causes. Two salient examples are the asteroid-impact hypothesis for the
extinction of the dinosaurs, which explains the fossil record of the dino-
saurs in terms of the impact of a large asteroid, and the “big-bang” theory
of the origin of the universe, which explains the puzzling isotropic three-
degree background radiation in terms of a primordial explosion. Such
work is significantly different from making a prediction and then artifi-
cially creating a phenomenon in a laboratory.

Scientists are well aware of the differences between experimental and
historical science vis-à-vis the testing of hypotheses. Indeed, it is some-
times a source of friction. Experimentalists have a tendency to disparage
the claims of their historical colleagues, contending that the support of-
fered by their evidence is too weak to count as “good” science. A telling
example is the startling number of physicists and chemists who attack
neo-Darwinian evolution on the grounds that it hasn’t been adequately
“tested.”1 The most sweeping condemnation of historical science, how-
ever, comes from Henry Gee, an editor of the prestigious science journal
Nature. In Gee’s words “they [historical hypotheses] can never be tested
by experiment, and so they are unscientific. . . . No science can ever be
historical” (2000, 5–8). In other words, for Gee, a genuine test of a hy-
pothesis requires classical experimentation.

Philosophers of science have pretty much ignored the simmering con-
troversy among scientists over the epistemic status of historical claims.
Aware that experiment plays different roles in science, skeptical about the
existence of a single method for all of science, and unable to provide an
epistemically satisfying account of the rationality and objectivity of any
scientific practice, philosophers have been reluctant to generalize, let alone
make normative judgments, across different disciplines. Moreover, those
philosophers who have cast an eye on historical research have spent most
of their time on evolutionary biology and archaeology, where issues about
teleology or human agency predominate. As a consequence, historical
methodology is often characterized (e.g., Goode 1977; Kitcher 1993, 18–
34) in terms of narrative histories. Analysis in terms of narrative histories
does not, however, do justice to historical work in disciplines such as
astronomy and geology that, like experimental physical science, do not
involve even a prima facie appeal to purposes or ends. A more general
understanding of the methodology of historical science and its differences
from classical experimental science is badly needed.



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 476

2. These regularities may be statistical, as in quantum mechanics, and they may or may
not be causal; experimental scientists are interested in testing functional regularities
(e.g., the Boyle-Charles’ law for ideal gases), as well as causal regularities.

3. I am using the term “auxiliary assumption” very broadly to include any assumption
whose falsity could be used to salvage a hypothesis in the face of a failed prediction.
Thus on my usage, auxiliary assumptions include assumptions about the particulars of

One of the purposes of this paper is to sketch such an account. As we
shall see, scientists engage in two very different patterns of evidential rea-
soning, and one of these patterns predominates in historical research and
the other in classical experimental research. These differences in evidential
reasoning lie at the heart of the charge that historical science is inferior to
experimental science vis-à-vis the testing of hypotheses. But, as I shall also
show, it is not an accident that historical research emphasizes one pattern
and experimental research the other, nor is it an accident that some in-
vestigations utilize both. Using examples from a wide variety of scientific
disciplines, I show that these differences in evidential reasoning are un-
derwritten by an objective and pervasive feature of nature, namely, a time
asymmetry of causation between present and past events, on the one hand,
and present and future events, on the other. Because each practice is tailored
to exploit the information that nature puts at its disposal, and the character
of that information differs, neither practice may be held up as more objec-
tive or rational than the other.

2. “Classical” Experimental Science. As Ian Hacking (1983, 149–166) has
emphasized, experiment plays a variety of roles in science besides the test-
ing of hypotheses. Nevertheless, there is little doubt that a significant por-
tion of experimental work is devoted to testing hypotheses in controlled
laboratory settings. My central concern in this paper is with this (what I
call the “classical”) role of experiment since it provides the paradigm to
which historical research is often compared unfavorably. Due to space
limitations, I shall focus on simple, idealized experiments, ignoring the
complications (see Franklin 1999, 13–38) that occur when one is working
with fancy equipment, dealing with experiments that cannot (for practical
or theoretical reasons) be “repeated” under controlled conditions, etc.

The hypotheses investigated in classical experimental science postulate
regularities among event-types.2 A test condition C is inferred from the
hypothesis and a prediction is made about what should happen if C is
realized (and the hypothesis is true). This forms the basis for a series of
controlled experiments.

In many of these experiments, C is held constant (repeated) while other
experimental conditions are varied. When this activity is preceded by a
failed prediction in an earlier experiment, it resembles the activity fa-
mously condemned by Popper (1963), namely, an ad hoc attempt to save
a hypothesis from refutation by denying an auxiliary assumption.3 But



!"#$% &. !%&%"'(477

an experimental situation (e.g., instruments are working properly, there are no inter-
fering conditions), as well as the more traditional theoretical assumptions about instru-
mentation, etc.

there is an alternative interpretation: It may be viewed as an attempt to
protect the target hypothesis from false negatives. Now, admittedly, the
distinction between protecting a hypothesis from falsification come-what-
may and protecting it from misleading disconfirmations seems subtle. But
it is not trivial. A false negative does not constitute a counter-example to
a hypothesis. Moreover, insofar as an experimental test of a hypothesis
involves an enormous number of auxiliary assumptions, the possibility of
false negatives is very real. Hence it makes good sense to control for them.
Each “control” represents a methodological rejection of an auxiliary as-
sumption about a previous experimental situation. These methodological
rejections of auxiliary assumptions amount to more than idle a priori
speculations. They are incorporated into actual experiments. In other
words, they aren’t just ad hoc attempts to salvage a hypothesis.

Significantly, the same process of holding C constant while varying
extraneous conditions often occurs upon a successful test of a hypothesis.
In this case, however, the activity resembles the sort of activity acclaimed
by Popper, viz., an attempt to falsify the target hypothesis. A little reflec-
tion, however, reveals that this can’t be what is going on. In the first place,
such tests are not “risky” in Popper’s demanding sense. The hypothesis
has survived similar tests and no one expects it to fail under slightly dif-
ferent conditions. Besides, even if it does, the hypothesis won’t automat-
ically be rejected. Viewed from this perspective, it seems more plausible to
interpret the activity as an attempt to protect the target hypothesis from
false positives. This interpretation is reinforced when one considers that
C itself may be removed in order to evaluate whether extraneous condi-
tions might have been responsible for the successful results of earlier ex-
periments.

Let us pull this all together. Much of the activity that goes on in classical
experimental science may be interpreted as attempts to protect the target
hypothesis from misleading confirmations and disconfirmations. This is
subtly but importantly different from attempting to falsify it, or to save it
from falsification by ad hoc means. For the falsification attempts are aimed
at auxiliary assumptions instead of the hypothesis under investigation. It
is important to keep in mind that this characterization of classical exper-
imental science is meant to capture an ideal, something to which experi-
mental scientists aspire, even though they may fail to realize it; as noted
earlier, it is sometimes difficult to repeat an experiment or to control for
suspicious auxiliary assumptions.

My account of classical experimental science bears a superficial resem-



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 478

4. Lakatos suggested that we think of scientific theories as divided into three compo-
nents: a “hard core,” a “protective belt,” and a “positive heuristic.” The positive heu-
ristic tells scientists how to respond to failed predictions (“anomalies”) by revising the
protective belt of auxiliary assumptions while keeping the hard core (which often in-
cludes the hypothesis under examination) intact.

blance to Lakatos’ (1970, 115–138) “sophisticated falsificationism.” What
I have in mind is nonetheless different. Lakatos was concerned with un-
derstanding how scientists protect their theories from refutations.4 He paid
little attention to the fact that scientists also protect their hypotheses from
misleading confirmations. Experimentation does not grind to a halt with
a successful experiment. Scientists continue to fiddle with extraneous con-
ditions while repeating C. They also try removing C while holding extra-
neous conditions constant. These two activities play just as prominent a
role in actual experimental practice as attempts to protect hypotheses from
false negatives. When one considers the vast number of additional con-
ditions (known and unknown) that might affect the outcome of an exper-
iment independently of the truth of the hypothesis, all three of these ac-
tivities make good sense. It is thus hardly surprising that experimental
scientists engage in them when they can.

My sketch of classical experimental science has the advantage of rec-
onciling the almost passionate devotion still expressed by many scientists
for falsificationism with the puzzling fact that they rarely reject their target
hypotheses in the face of failed predictions. They are engaging in system-
atic, extended experimentation that sometimes resembles an attempt to
falsify a hypothesis and sometimes resembles an attempt to protect a hy-
pothesis from falsification, but is really aimed at something quite different,
namely, minimizing the very real possibility of misleading confirmations
and disconfirmations in concrete laboratory settings. The historical ten-
dency of philosophers to take the isolated experiment as the descriptive
unit of experimental research has obscured this character of classical ex-
perimental science. For if one focuses on the construction and evaluation
of a solitary experimental test of a target hypothesis, it seems as if re-
searchers are either trying to falsify the hypothesis or save it from falsifi-
cation. Viewed from a temporally extended perspective, however, things
look quite different. The experimental evaluation of a hypothesis involves
a series of experiments, each one designed in light of the results of previous
experiments. In the face of an ostensibly disconfirming result, auxiliary
assumptions are modified. Similarly, auxiliary assumptions are also mod-
ified in the face of an ostensibly confirming result, and the test condition
itself is eventually removed. In the absence of a series of experiments of
this character, most researchers are reluctant to submit their work to a
peer reviewed science journal.



!"#$% &. !%&%"'(479

5. This is discussed in DiGregorio (1997, 167–174). A well known planetary scientist
who does not wish to be in any way associated with Levin’s position has also confirmed
this in a private conversation.

It is important to be clear about the nature of the interdependency
among the experiments in an experimental program. The results of earlier
experiments guide the design of later experiments. The 1976 Viking Lander
missions to Mars provide a particularly salient illustration of what hap-
pens when scientists try to design an experimental program wholly in ad-
vance of any data. The Viking Landers were designed to perform robotic
experiments testing the hypothesis that Mars had microbes living in its
soil. Of particular interest are the “labeled release” (LR) experiments,
which generated the most consistently positive results of the three classes
of metabolic experiments; for more detail, see Klein et al. 1992, 1221–
1233. The initial results of the LR experiment were positive. When the
Martian soil sample in the test chamber was injected with a radioactively
labeled bacterial nutrient solution it started evolving radioactive 14CO2,
just what one would expect from terrestrial soils containing microbes.
Moreover, when the controlled experiment (determined in advance of any
data) was performed and the Martian soil sample was “sterilized” by heat-
ing it to 160o C for 3 hours, the reaction stopped, strongly suggesting that
the initial result really had been biological; this experiment amounted to
a test for a false positive. But the data from the LR experiment were not
yet all in. When the same soil sample was subjected to another experiment
and given a “second helping” of radioactive nutrients several days later,
after the initial reaction had leveled off, the anticipated burst of new ac-
tivity (from hungry Martian microbes) failed to occur. Even more mys-
teriously, 14CO2 left over from the earlier reaction began disappearing.
Scientists were baffled. No one had anticipated results like these.

As the Viking Landers fell permanently silent, disagreement over the
interpretation of the LR experiments became acrimonious, with alterna-
tive theoretical explanations (Oyama et al. 1976; Levin and Straat 1976)
being advanced for the puzzling results. To this day, two of the original
investigators (Levin and Straat) still insist that they found life. A majority
of the scientific community, however, disagrees, contending that the Mar-
tian surface contains a strong oxidant. Various candidates, e.g., hydrogen
peroxide (Oyama et. al. 1976), high oxidation states of iron (Tsapin et al.
2000), have been proposed for the oxidant. Nevertheless, there is no em-
pirical evidence that the Martian surface is strongly oxidizing.5 The basic
problem remains. The LR experiments and their controls produced results
that no one had anticipated and there was no possibility of performing
suitable additional experiments to adjudicate among the many auxiliary
assumptions that became the focus of heated speculation. In retrospect, it



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 480

6. I am appropriating this expression from historical scientists. Talk about a “smoking
gun” is rampant in informal and popular works by historical researchers. As an ex-
ample, in a discussion of the asteroid impact hypothesis for the extinction of the di-
nosaurs, James Powell refers to the “tiresome metaphor of ‘smoking gun’” (1998, 115).
It should be kept in mind, however, that historical scientists use the term very loosely,
and in what follows, I am providing it with a technical meaning that doesn’t always
coincide with its use among historical scientists.

was naı̈ve of designers of the experiment to think that they could determine
in advance of any experimental data what controls would be adequate to
rule out false positives and false negatives.

Although my focus has been on the (idealized) practice of an individual
researcher (and her co-workers) testing a hypothesis, I am not claiming
that her decision to accept or reject a hypothesis is based solely upon her
own experimental results, nor am I denying the important Kuhnian insight
(1970) that many of the factors involved in a scientific community’s de-
cision to accept or reject a hypothesis are based upon sociological and
psychological considerations. I am merely trying to characterize what the
scientific community expects of a “good” experimental researcher when
she goes about testing a hypothesis in her lab, and I am claiming that it
is best understood in terms of an extended series of interdependent exper-
iments, as opposed to the solitary experiment.

3. Historical Science. In light of the above discussion, let us turn to the
practice of historical science. In the prototypical scenario, an investigator
observes puzzling traces (effects) of long-past events. Hypotheses are for-
mulated to explain them. The hypotheses explain the traces by postulating
a common cause for them. Thus the hypotheses of prototypical historical
science differ from those of classical experimental science insofar as they
are concerned with event-tokens instead of regularities among event-types.
This helps to explain the narrative character of many historical explana-
tions. The complexity of the causal conditions and the length of the causal
chain (connecting the cause to its current traces) bury the regularities in a
welter of contingencies. Accordingly, it is hardly surprising that historical
explanations often have the character of stories that, lacking reference to
specific generalizations, seem inherently untestable. Nonetheless, it would
be a mistake to conclude that hypotheses about the remote past can’t be
“tested.”

Traces provide evidence for past events just as successful predictions
provide evidence for the generalizations examined in the lab. Instead of
inferring test implications from a target hypothesis and performing a series
of experiments, historical scientists focus their attention on formulating
mutually exclusive hypotheses and hunting for evidentiary traces to dis-
criminate among them. The goal is to discover a “smoking gun.”6 A smok-



!"#$% &. !%&%"'(481

ing gun is a trace(s) that unambiguously discriminates one hypothesis from
among a set of currently available hypotheses as providing “the best ex-
planation” of the traces thus far observed.

Wegener’s hypothesis of continental drift provides a salient illustration
(for more detail, see Hallam 1973, 135–183). When it was first advanced
in 1912, the hypothesis provided a unified account of a large number of
puzzling features of the surface of Earth, e.g., the complementary shapes
of the Atlantic coasts of Africa and South America, and similarities in
geological formations and fossil records on opposite sides of the Atlantic
Ocean. The competing hypothesis, the contractionist theory, held that
Earth’s crust moves only vertically, a result of gradual contraction as its
interior cools. The contractionist hypothesis didn’t provide as unified an
account of the known traces as Wegener’s hypothesis. But Wegener’s hy-
pothesis had a major defect. There was no known causal mechanism for
horizontal continental motion. This was partially remedied in 1960 by
Princeton geologist Harry Hess’s hypothesis of sea floor spreading, which
was formulated in light of new discoveries derived from advances in geo-
logical and geophysical oceanography; these discoveries included the
global mid-oceanic ridge system, the apparent youth of the sea floor, and
the circum-Pacific island arc-trench system with its numerous volcanoes
and powerful earthquakes. The subsequent discovery of alternating bands
of reversed magnetism, spreading out symmetrically on both sides of the
volcanically active Mid-Atlantic Ridge provided the smoking gun for sea
floor spreading. The magnetic stripes provided compelling evidence that
the Earth’s crust moves horizontally, carrying the continents along with
it. But widespread scientific acceptance of sea floor spreading had to wait
a few more years until a geophysical mechanism for sea floor spreading
was worked out in the theory of plate tectonics.

The evolution of Wegener’s hypothesis (from continental drift to sea
floor spreading to plate tectonics) underscores some crucial characteristics
of successful historical work. First, as Philip Kitcher (1989, 430–432) and
others have emphasized, the best scientific explanations are unifying. In
successful historical science the focus is on the unity that a hypothesis
provides for a diverse body of puzzling traces. One of the great strengths
of the plate-tectonic hypothesis is the large number of ostensibly indepen-
dent phenomena that are united under a single explanation. Second, as
Salmon (1984, ch. 5) has insisted, good explanations situate observations
(in this case, traces) within a causal framework. This was the central prob-
lem with Wegener’s original hypothesis. From the beginning, it provided
more unity for the observed traces than the contractionist hypothesis. But
there was no known physical mechanism for producing horizontal conti-
nental motion. Widespread acceptance of Wegener’s ideas had to wait for
the development of the geophysical theory of plate tectonics. In short,



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 482

successful historical hypotheses explain traces by unifying them under a
consistent causal story.

The greater and more diverse the body of traces that a postulated causal
mechanism unifies, the better the explanation. A smoking gun is a trace
(or subcollection of traces) that (so-to-speak) cinches the case for a par-
ticular causal story. A smoking gun does not, however, uniquely determine
a hypothesis outside the context of a set of specific, competing hypotheses;
it merely establishes that one of them is superior when it comes to causally
explaining the traces thus far observed. Since many of the pertinent traces
will be equally well explained by the competing hypotheses, a smoking
gun will not include all of them. Furthermore, it is always possible that
future observations or theoretical developments will depose a smoking gun
and that another hypothesis (new or old) will attain the status of the best
explanation. Moreover, some of the factors that go into transforming a
trace(s) into a smoking gun for a particular hypothesis may be sociological
or psychological, as well as theoretical and empirical; historical research
is just as subject to Kuhnian influences as experimental research. In short,
a “crucial” test of a hypothesis is no more possible in historical science
than in experimental science. But this doesn’t mean that historical re-
searchers can’t procure supporting evidence for their hypotheses any more
than it means that experimentalists can’t procure supporting evidence for
their hypotheses.

The history of the debate over the extinction of the dinosaurs provides
a good illustration of the crucial role played by a smoking gun in adju-
dicating among rival hypotheses. Prior to 1980 there were many different
explanations for the demise of the dinosaurs, including contagion, global
climate change, volcanism, and asteroid impact. Luis and Walter Alvarez’s
discovery (1980) of unusually high concentrations of the element iridium
in the K-T (Cretaceous-Tertiary) boundary (ten parts per billion vs. back-
ground levels of about three-tenths of a part per billion) focused attention
on volcanism or an asteroid impact, since these were the only plausible
mechanisms for the presence of so much iridium in a thin layer of Earth’s
crust; higher concentrations exist in Earth’s mantle and in asteroids. The
subsequent discovery (Bohor, Foord et al. 1984) of extensive deposits of
shocked quartz in the K-T boundary clinched the case for most geologists
because, as was soon established (Alexopoulos et al. 1988), shocked quartz
of the sort found in the K-T boundary is not associated with volcanism;
it is found in only two places on Earth, craters produced by asteroid im-
pacts and the sites of nuclear explosions. Geologists began searching for
a crater of the right size and age; they eventually found Chicxulub Crater,
which overlaps the Yucatan peninsula and is about 65 million years old
and 200–300 km across. It was generally admitted, however, that failure
to find the crater wouldn’t count heavily against the hypothesis since the



!"#$% &. !%&%"'(483

active geology of Earth might have obliterated all traces, particularly if
the impact had occurred in deep ocean. The combination of iridium and
shocked quartz were enough to convince most members of the geological
community that a large asteroid had slammed into Earth 65 mya.

While it was widely conceded that the anomalous iridium and shocked
quartz provided a “smoking gun” for the impact of a large asteroid, pa-
leontologists (e.g., Clemens et al. 1981) remained unconvinced that the
impact explained the extinction. More research was needed to establish a
causal link between the impact and the extinction. The extinction event
had to be word-wide and geologically instantaneous. The available paleo-
biological data were very imprecise, unable to distinguish global events
occurring within a period of a few years from events that took place at
different times throughout intervals of 10,000 to perhaps 500,000 years.
Peter Ward’s (1990) meticulous studies of the fossil record of the ammo-
nites (which is much more extensive than that of the dinosaurs) were piv-
otal in marshalling support among paleontologists for the claim that the
Cretaceous extinctions were rapid and global. The asteroid-impact hy-
pothesis became the widely accepted explanation for the extinction of the
dinosaurs. For of the available hypotheses, it provided the greatest causal
unity to the diverse and puzzling body of traces (fossil record of the di-
nosaurs, fossil record of the ammonites, etc., and iridium anomaly,
shocked quartz, Chicxulub Crater, etc.).

So how does prototypical historical research compare to classical ex-
perimental research? Historical scientists are just as enamored with falsi-
ficationism as experimentalists; as three eminent geologists counsel in a
recent textbook discussion of the extinction of the dinosaurs, “a central
tenet of the scientific method is that hypotheses cannot be proved, only
disproved” (Kump, Kasting, and Crane 1999, 201). Nevertheless, there is
little in the practice of historical science that resembles what is prescribed
by falsificationism. The search for a smoking gun is a search for supporting
evidence for a hypothesis. Moreover, a trace that comprises a smoking
gun may not (considered just in itself) provide evidence against a com-
peting hypothesis. In the context of the contagion hypothesis, for instance,
the presence of shocked quartz in the K-T boundary seems merely irrel-
evant to the extinction of the dinosaurs. This is not to deny that a search
for a smoking gun may turn up evidence against a hypothesis. The point
is that it need not. One can establish that a hypothesis provides a more
unified causal explanation than another without procuring direct evidence
against the latter.

Furthermore, there is little in prototypical historical research that re-
sembles attempts to control for false positives and false negatives. Gen-
erating competing hypotheses is not analogous to entertaining auxiliary
assumptions. The truth of an auxiliary assumption is independent of the



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 484

truth of the target hypothesis, which is why a target hypothesis may be
saved (in the face of a failed prediction) by rejecting an auxiliary assump-
tion. In contrast, competing historical explanations are incompatible.
Moreover, if there is a smoking gun for an historical hypothesis, it already
exists. Smoking guns are not produced by systematically varying some
conditions while holding others constant. They are uncovered in the un-
controlled world of nature by fieldwork.

This is not to deny that prototypical historical research often involves
laboratory work. It is important, however, to be clear about what is ac-
tually being investigated in the lab. Most often it is the evidentiary traces,
which frequently require sharpening or analysis in order to be identified
and properly interpreted. As an example, speculation that life goes back
3.8 billion years rests upon laboratory analysis of carbon isotope ratios in
grains of rock as small as 10 lm across weighing only 20x10-15 g (Mojzsls
et al. 1996, 56). Sometimes, however, it is a hypothesis bearing a tenuous
logical relation to the target hypothesis. A good example is the 1953
Miller-Urey experiments (Miller 1953), which were touted as an experi-
mental test of the hypothesis that life on Earth originated in a “primordial
soup” but really amounted to a test of the supposition that some of the
most basic building blocks of life (amino acids) can be produced by elec-
trical discharges on a mixture of methane, hydrogen, ammonia, and water.
This is not to deny that the Miller-Urey experiments provide some support
for the hypothesis that life on Earth began in an electrified primordial
soup. The support is not analogous, however, to that offered by a con-
trolled experiment for a target hypothesis. This is brought out clearly by
the fact that scientists now know that amino acids can be produced under
a wide variety of different conditions (Chyba and Sagan 1992). Further-
more, most scientists currently believe that the origin of life on Earth is
incompatible with the conditions of the Miller-Urey experiment; it is
thought that the Earth’s early atmosphere did not contain abundant meth-
ane or ammonia, and that life probably began on or under the sea floor,
near a volcanic vent (Pace 1991). The problem with the Miller-Urey ex-
periment is that the logical relation between the hypothesis actually tested
in the lab and the target hypothesis (about the origin of life on Earth) is
very convoluted, winding through numerous highly speculative assump-
tions, ranging from conditions on early Earth to biochemical possibilities
for producing amino acids and whole cells. This is fairly typical of exper-
imental work associated with hypotheses about the remote past.

I have been describing two different patterns of evidential reasoning,
put succinctly, from causes (test conditions) to effects, with the concomi-
tant worries about ruling out false positives and false negatives, and from
effects (traces) to causes, with the concomitant worries about ruling out
alternative explanations. As we have seen, although one pattern of rea-



!"#$% &. !%&%"'(485

soning predominates in historical research and the other in experimental
work, there is overlap. Historians sometimes reason like experimentalists,
and vice versa. This is not an accident. Which pattern of reasoning a
scientist employs depends upon her epistemic situation, and although the
epistemic situation of an historian typically differs from that of an exper-
imentalist, there are some notable exceptions.

Nature sometimes repeats herself, presenting researchers with multiple
examples of a given type of historical phenomenon. In such cases, the body
of evidence available to researchers may resemble what an experimental
program would provide, and they may exploit this fact by formulating
generalizations analogous to those formulated by experimentalists. Stars
known as “Cepheid variables” provide a perspicuous example. The length
of the period of a Cepheid variable (time interval between two successive
states of maximal brightness) is lawfully correlated (the Leavitt-Shapley
law) with the magnitude of its intrinsic luminosity. Although one cannot
perform controlled experiments on Cepheid variables, one can search for
them with powerful telescopes and ascertain whether those that are dis-
covered obey the Leavitt-Shapley law. Insofar as quite a few Cepheid vari-
ables have been identified, the body of this work resembles what might be
achieved by controlled experimentation.

There are important differences, however. One cannot produce Cepheids
with specific periods (the “test condition”) at will, let alone control for
possibly relevant extraneous conditions. Perhaps the Cepheid variables
thus far observed are unusual in some special way. An astronomer may
even suspect that a certain stellar condition is pertinent. But she can’t
remove or introduce it. She can only patiently scan the heavens for its
traces. Viewed from this perspective, she is in the same position as a pa-
leontologist hunting for evidence that the great Permian extinction was
also caused by the impact of a large asteroid, the main difference being
the lack of much evidence—good continuous exposures of rock—dating
from the end of the Permian. The point is, without the ability to manip-
ulate suspect conditions, one is at the mercy of what nature just happens
to leave in her wake; sometimes she is generous and sometimes she is
stingy, but the bottom line is that you can’t fool with her. When nature is
generous, however, there is little doubt that the body of evidence used to
support a hypothesis may more closely resemble that provided by classical
experimental research than by prototypical historical research, with its
emphasis on finding pivotal evidence—a smoking gun—for discriminating
a hypothesis from its competitors.

Just as historians sometimes reason like experimentalists, so experi-
mentalists sometimes reason like historians. In the face of a failed exper-
imental test of a target hypothesis, experimentalists entertain different pos-
sibilities for denying auxiliary assumptions, and their reasoning resembles



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 486

that of a historian choosing from among alternative explanations for puz-
zling traces. But the subsequent work (removing the suspect condition
while holding other conditions constant) bears little resemblance to what
goes on in an historical search for a smoking gun. Historical researchers
searching for a smoking gun are stuck with what nature has already pro-
vided; they must be clever enough to ferret out incriminating evidence that
is often well hidden in the messy, uncontrollable world of nature. Simi-
larly, like historians, experimentalists are sometimes faced with unex-
pected and puzzling effects. Such laboratory phenomena may be produced
by accident, during a test of a hypothesis, or deliberately, out of curiosity.
For, as Hacking discusses (1983, 155–159), scientists often play around
with their equipment to see what it will do, sometimes just for the sake of
generating interesting phenomena and other times because they have a
vague suspicion that they wish to explore. A well-known example of the
latter is Faraday’s work on electromagnetism; he suspected that magnetic
fields could induce electric effects, and deliberately searched for and found
evidence of this. Henri Becquerel’s discovery of radiation provides a sa-
lient example of the accidental discovery of a new phenomenon; he dis-
covered an intense image on a photographic plate that had been exposed
to uranium salts on an overcast day and then haphazardly tucked away
in a dark drawer. When faced with a new phenomenon, laboratory sci-
entists explore it, trying to reproduce it and speculating about its cause.
That is, like historical scientists, they generate competing hypotheses
about its source and search for additional evidence.

Once experimentalists have determined that a phenomenon is repeat-
able, however, they begin formulating specific generalizations about it and
testing them individually by manipulating test conditions in the manner
of classical experimental science. Tom Cech’s Nobel Prize winning dis-
covery that RNA can behave as a catalyst (see Zaug and Cech 1986)
provides a good example of this transition. While working on an unrelated
hypothesis, he was surprised to discover catalytic products in a solution
of RNA that supposedly contained no proteins. He formulated competing
explanations for this puzzling phenomenon (e.g., the solution was contam-
inated by proteins after all, RNA can behave as a catalyst), and then
proceeded to test each of them in the laboratory in the classical manner
by fiddling with suspect auxiliary assumptions (controlling for unsus-
pected protein contamination) and removing the test condition (the RNA)
and seeing whether he still got the catalytic products.

In summary, although there is overlap, there are nonetheless funda-
mental methodological differences between historical science and experi-
mental science vis-à-vis the testing of hypotheses. These differences in
methodology reflect the fact that experimentalists and historians typically
find themselves in very different epistemic situations. Experimentalists are



!"#$% &. !%&%"'(487

primarily concerned with evaluating repeatable generalizations. Their re-
search is focused on generating predictions from a single (sometimes com-
plex) hypothesis, and manipulating repeatable test conditions in a lab
while controlling for extraneous factors that might produce false positives
and false negatives. Scientists engaged in prototypical historical work, on
the other hand, are primarily concerned with evaluating hypotheses about
particular past events. They cannot reproduce these events in a lab. They
can, however, look for present-day traces of them, and search for a smok-
ing gun that unambiguously sets apart one hypothesis as the best among
the currently available explanations for the traces thus far observed. These
differences in methodology do not, however, support Gee’s charge that
prototypical historical research is inferior to classical experimental work.
For, as we shall see, the patterns of evidential reasoning upon which they
are founded are epistemically underwritten by a corresponding difference
in nature, namely, a time asymmetry of causation.

4. The Asymmetry of Overdetermination. Many events (broadly construed
to include states) are causally connected in time in a strikingly asymmetric
manner. Consider, for instance, the shattering of a window by a baseball.
It is easy to explain the aftermath (a baseball on the floor of the bedroom
and many pieces of glass strewn about on either side of an empty window
frame) in terms of a single local event, namely, the impact of the baseball
on a windowpane. But now run the process backwards in time. Dozens of
widely scattered pieces of glass suddenly twitch and then are rapidly
launched into the air, converging on the empty window frame. Meanwhile
the baseball also begins moving. It too is ejected into the air, and flies
through the empty window frame just before all the pieces of glass reach
it. As the baseball clears the frame, each piece of glass simultaneously finds
a spot that is precisely its shape and size. The result is an utterly seamless
pane of glass. From this time reversed perspective, the events involved in
the shattering of the window seem baffling, requiring the concerted but
independent action of a baseball and many separate pieces of glass. And,
indeed, we never see such sequences in nature: broken windows don’t sud-
denly reconstitute themselves from widely scattered pieces of glass.

That this time asymmetry of nature applies to more than broken win-
dows can be appreciated by considering the difficulty involved in com-
mitting the perfect crime. Footprints, fingerprints, particles of skin, dis-
turbed dust, and light waves radiating outward into space must be
eliminated. Moreover, it isn’t enough to eliminate just a few of these traces.
Anything you miss might be discovered by a Sherlock Holmes and used
to convict you. Finally, each trace must be independently erased. You
cannot eliminate the footprints by removing particles of skin or, for that
matter, one footprint by removing a different one. In stark contrast, pre-



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 488

venting a crime from occurring is easy: don’t hit the baseball; don’t fire
the gun. In other words, erasing all traces of an event before it occurs is
much easier than erasing all traces of it after it occurs.

David Lewis (1991, 65–67) has dubbed this feature of the world “the
asymmetry of overdetermination.” The basic idea is that events leave wide-
spread, diverse effects (such as pieces of glass, particles of skin, etc.). Any
one of a large number of contemporaneous, disjoint combinations of these
traces is sufficient (given the laws of nature) to conclude that the event
occurred. One doesn’t need every shard of glass in order to infer that a
window broke. A surprisingly small number of appropriately dispersed
fragments will do. The overdetermination of causes by their effects makes
it difficult to fake past events by engineering the appropriate traces since
there will typically be many other traces indicating fakery. Similarly, as
Lewis notes (1991, 66), there will usually be traces indicating that a ques-
tionable trace wasn’t faked. Good criminal investigators are very aware
of such possibilities: Is the handwritten ransom note left at the Ramsey
residence genuine or designed to throw detectives off the trail of the real
murderer? This is not to deny that traces may be so small, far flung, or
complicated that no human being could ever decode them. All that is
required for Lewis’ thesis of overdetermination, however, is that they exist.

In contrast, the causal predecessors of an event rarely overdetermine
it. A good example is a short circuit that “causes” a house to burn down.
Take away the short circuit and the house wouldn’t have burned down;
the short circuit “triggered” the fire. But the short-circuit isn’t sufficient
for the occurrence of a fire. Many other factors are also required, e.g., the
presence of flammable material, absence of sprinklers. The absence of any
one of these additional factors would also have prevented the fire, even
supposing that the short circuit had occurred. In other words, the total
cause of the fire includes more than the short circuit. Local events (such
as the short circuit) which are normally identified as the causes of later
events (a house burning down) underdetermine them; considered just in
themselves, they are not enough (even given the laws of nature) to guar-
antee that their reputed effects occur. This is the other side of the asym-
metry of overdetermination. While erasing all traces of an event requires
many separate interventions, a single intervention (and there are usually
numerous possibilities) is normally enough to prevent it from occurring
in the first place.

The underdetermination of effects by their putative causes is both ep-
istemic and causal; the fire is epistemically and causally underdetermined
by the short circuit. The overdetermination of causes by their effects, on
the other hand, is (strictly speaking) only epistemic. Although one may
infer that the window broke from any one of a large number of subcol-
lections of shards of glass, the shards are not part of the cause of the



!"#$% &. !%&%"'(489

breaking of the window; they are its effects. But although effects cannot
causally overdetermine causes, there is no reason why causes couldn’t
causally overdetermine effects, and indeed this sometimes happens. The
well-worn example is a firing squad, where several bullets simultaneously
pierce the victim’s heart. Their scarcity underscores the fact that the asym-
metry of (epistemic) overdetermination is ultimately founded on a time
asymmetry of nature.

There is little agreement about the physical source of the asymmetry of
overdetermination. Examples such as the shattering of a window are com-
monly attributed to the second law of thermodynamics, which (statistically
interpreted) says that the natural tendency of physical systems is to move
from more to less ordered states. The asymmetry of overdetermination
extends, however, to phenomena that do not obviously admit of a ther-
modynamic explanation. Consider Popper’s (1956, 538) well-known ex-
ample of dropping a stone into a still pool of water. Expanding concentric
ripples spread outwards from the point of impact. It is easy to explain
these ripples in terms of a stone entering the water at a small region of the
surface of the pool. Indeed, one can pinpoint where the stone entered by
examining a small segment of the pool’s surface. But now consider elim-
inating all traces of the impact. An enormous number of separate and
independent interventions are required all over the surface of the pool.
Similarly, try explaining the time reversed process. Ripples, which ex-
panded outwards from the point of impact, now contract inwards to the
point of impact. But there is no center of action to explain the simulta-
neous and coordinated behavior of the individual water molecules in-
volved. From this time reversed perspective, the contracting concentric
waves seem to be a miracle; we can understand them causally only by
running the process in the other direction (forwards) in time.

Popper’s example of the stone and the pool illustrates a phenomenon
known as the asymmetry of radiation. Although the asymmetry of radi-
ation is traditionally associated with electromagnetic radiation (light, ra-
dio waves, etc.), it characterizes all wave-producing phenomena, including
disturbances in water and air. The asymmetry originates in the fact that
waves (whether of water, sound, light, etc.) invariably spread outwards
rather than inwards. The asymmetry of radiation is thus very widespread
in nature.

It is tempting to suppose that the asymmetry of radiation is connected
to the asymmetry of thermodynamics. For our purposes, however, it
doesn’t matter how the asymmetry of radiation is related to that of ther-
modynamics. What matters is that both asymmetries are objective physical
features of the universe. There is widespread agreement about this. Indeed,
the only one who comes close to denying it is Price, who contends (1996,
155–161) that the asymmetries characterize only the macroscopic realm of



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 490

7. I am indebted to Sheralee Brindell, a self-professed fan of “cheesy” detective novels,
for emphasizing the similarities between historical research and a criminal investigation.

human experience. But even Price admits (e.g., 1996, 76) that it is a physi-
cal fact that our universe contains numerous, large, organized concentra-
tions of energy in the form of galaxies, stars, rocks, pools, etc., which give
rise to macroscopic asymmetries of radiation and thermodynamics. This
is enough for our purposes because this is the level at which scientific
investigations (in the field or lab) are ultimately carried out.

This brings us back to the practice of science. Historical researchers
investigating particular past events cannot test their hypotheses by per-
forming controlled experiments. But this doesn’t mean that they cannot
procure empirical evidence for them. Because of the asymmetry of over-
determination, there are usually an enormous number of subcollections
of the effects of a past event that are individually sufficient (given the right
theoretical assumptions) to infer its occurrence. The trick is finding them.
Many of these overdetermining traces (e.g., a splinter of glass, a footprint)
occupy small, local regions of space, bringing them within the limited
range of human sensory experience. This places scientists investigating the
remote past in the position of criminal investigators.7 Just as there are
many different possibilities for catching a criminal, so there are many
different possibilities for establishing what caused the demise of the di-
nosaurs, the origin of the universe, etc. Like criminal investigators, his-
torical scientists collect evidence, consider different suspects, and follow
up leads. More precisely, they postulate differing causal etiologies for the
traces they observe, and then try to discriminate from among them by
searching for a “smoking gun”—a trace(s) that identifies the most plau-
sible culprit among the primary suspects. Unlike stereotypical criminal
investigations, however, a smoking gun for a historical hypothesis merely
picks out one hypothesis as providing the best explanation currently avail-
able; it need not supply direct confirming evidence for a hypothesis inde-
pendently of its rivals.

Although Lewis characterizes the asymmetry of overdetermination in
terms of sufficiency, it could turn out to be a probabilistic affair, with the
ostensibly overdetermining subcollections of traces lending strong but,
nevertheless, inconclusive support for the occurrence of their cause. Like
the determinism in Lewis’ original version, the probabilistic support of-
fered by collections of traces for hypotheses would be an objective feature
of the world. A body of traces would make its cause highly probable
independent of human knowledge, interests, or concerns. Researchers in
possession of a decisive body of traces might fail to recognize its signifi-
cance; they might not have formulated the correct hypothesis or they
might lack the theoretical understanding necessary to connect the traces



!"#$% &. !%&%"'(491

with the correct hypothesis. Developing a satisfactory probabilistic inter-
pretation of the asymmetry of overdetermination is beyond the scope of
this paper. Needless to say, there are many issues that need to be ad-
dressed, e.g., are the probabilities relative to more global background con-
ditions (thus requiring ceterus paribus clauses) or are they absolute? In
any case, however, human experience is consistent with either a probabi-
listic or a deterministic interpretation of the asymmetry of overdetermi-
nation. Just as experimental work is irremediably fallible (even in a deter-
ministic world) because of the uneliminable threat of unknown interfering
conditions, so the traces uncovered by field work are never enough to
conclusively establish the occurrence of a postulated past event, perhaps
because we haven’t discovered enough of them or perhaps simply because
there are no causally sufficient subcollections. But even supposing that it
is probabilistic, the asymmetry of (quasi) overdetermination helps to ex-
plain the methodology of historical research. It tells us that a strikingly
small subcollection of traces is enough to substantially increase the prob-
ability that a past event occurred, and that there are likely to be many
such subcollections. The existence of so many different reliable possibili-
ties for identifying past events provides the rationale for the historical
scientist’s emphasis on finding a smoking gun.

The empirical support currently enjoyed by the Alvarez hypothesis pro-
vides an excellent illustration of the evidential resources available to his-
torical researchers. In addition to shocked quartz and iridium, the K-T
boundary sediments have yielded microspherules (unusual microscopic
droplets of glass thought to have been produced by the rapid cooling of
molten rock that splashed into the atmosphere during the impact), fuller-
enes containing extraterrestrial noble gases, and extensive deposits of soot
and ash (measuring tens of thousands of times higher than normal levels).
Other than a catastrophic extraterrestrial impact, there is no known mech-
anism for producing these unusual and disparate traces. Had iridium been
absent from the K-T boundary, scientists would still have excellent
grounds (given currently accepted theoretical considerations) for conclud-
ing that a giant asteroid slammed into Earth 65 mya. The increasing num-
ber of scientists who believe that the great Permian extinction was also
caused by an asteroid impact underscores this point. Sediments at the
Permian-Triassic boundary lack an iridium anomaly but they contain a
diversity of other traces (most notably, extraterrestrial fullerenes) that are
difficult to explain in terms of any other known mechanism (Becker et al.
2001). Similarly, recent high-resolution studies of the fossil record of mul-
tiple species, e.g., studies of fossilized plant remains (see Archibald 1996),
are in agreement with Ward’s ammonite studies about the rapidity and
timing of the K-T extinctions. Even in the absence of Ward’s work, these
studies would provide strong support for a causal connection between the



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 492

extinctions and the impact. In other words, the body of traces thus far
uncovered is more than enough to unambiguously establish the superiority
of the Alvarez hypothesis vis-à-vis its competitors, and the existence of this
large and diverse body of supporting evidence is a consequence of an
objective physical feature of the universe, namely, the asymmetry of ov-
erdetermination.

The hard part of historical research is finding traces that are capable
of unambiguously discriminating among rival hypotheses. But even when
much time has passed and the traces have become very attenuated there
is always the possibility of discovering a smoking gun for a long past event.
In some cases, one may be able to infer a smoking gun directly from the
hypothesis under investigation. A good example is the theoretical work
done in the mid-1960s by Robert Dicke and his team of Princeton phys-
icists (see Kaufmann 1977, 267–277). They predicted that if the big-bang
theory for the origin of the universe was true the universe should contain
an isotropic, microwave background radiation a few degrees above ab-
solute zero. Wilson’s and Penzias’ subsequent discovery of the mysterious
3-degree background radiation was taken as providing pivotal evidence
for the big-bang theory over the steady state theory. Sometimes, however,
one just gets lucky and stumbles over a smoking gun as did the Alvarezes
in the case of the asteroid-impact hypothesis for the extinction of the di-
nosaurs; the existence of iridium and shocked quartz in the K-T boundary
was not predicted in advance of its discovery. In this context, I want to
emphasize that I am not claiming that every true historical hypothesis has
a smoking gun. It is unlikely but nonetheless possible for an event to leave
no traces; prime candidates are events occurring before the big-bang of
cosmology. Moreover, with the passage of time, traces of events become
more and more attenuated, and eventually may disappear altogether. Al-
ternatively, they may still be present but very degraded. Finding them may
require advances in technology. The discovery of the 3-degree background
radiation depended upon the development of very sensitive antennas for
communicating with satellites. Similarly, a particle accelerator (cyclotron)
was used in the discovery of the iridium in the K-T boundary. Finally,
with new evidence and new explanatory hypotheses, the status of a trace
as a smoking gun may change. Nevertheless, one can never rule out the
possibility of finding a smoking gun, and this is a consequence of an ob-
jective fact about nature, namely, most past events are massively overde-
termined by localized present phenomena.

The on-going debate over whether meteorite ALH84001 contains fos-
silized Martian life provides an excellent overview of how historical re-
searchers exploit the asymmetry of overdetermination in their efforts to
adjudicate among competing explanations. Discovered in the ice fields
of Antarctica in 1984 and subsequently determined to be from Mars,



!"#$% &. !%&%"'(493

ALH84001 contains a number of unexpected and very puzzling structural
and chemical features, including layered carbonate globules, iron sulfides,
PAHs, sausage-shaped (a.k.a. bacteria-shaped) structures, and “pris-
matic” (single domain) magnetite. These features would not be expected
to form under a single combination of geochemical conditions. Terrestrial
bacteria, however, produce them all. McKay, Gibson, and their team at
Johnson Space Center postulated that they had been caused by ancient
Martian microbes (see McKay, Gibson et al. 1996). The response of the
scientific community was cautious. While conceding that the evidence was
intriguing, most scientists nonetheless rejected the claim that it provided
strong support for Martian microbial activity. More research was needed.

Samples of ALH84001 (which weighed only 1.9 kg) were carefully doled
out to researchers with access to sophisticated analytical techniques and
equipment. As of this writing, ALH84001 is one of the most intensely
studied rocks on Earth. The results of many of these investigations have
been equivocal. As an example, analysis of some of the features (e.g., the
magnesium, calcium, and iron compositions found in the carbonate glob-
ules) suggests that they were formed at temperatures much too high
(! 650"C) for life as we know it (Kerr 1997), whereas analysis of other
features (analysis of two adjacent pyroxene grains in the “crushed zone”
of the meteorite) suggest otherwise (Kirschvink et al. 1997). The feature
in ALH84001 that has attracted the most attention, however, is the pris-
matic magnetite. Using electron microscopy and energy dispersive spec-
troscopy (McKay et al. 1996, 926), McKay and his team discovered tiny
(20–100 nm) crystals of magnetite in the carbonate globules. Approxi-
mately 25% of these crystals have the distinctive feature of being hexag-
onal in cross section and virtually free of chemical impurities. There are
no known nonbiotic processes that produce magnetite of this shape or
chemical purity. It is indistinguishable, however, from that produced in-
tracellularly by some magnetotactic terrestrial bacteria (see Thomas-Ke-
prta et al. 2000). It is for this reason that (in an unpublished 1998 “Position
Paper” widely distributed among scientists) McKay and his team referred
to the prismatic magnetite as a “smoking gun” (p. 5) for Martian life.

Nevertheless, the scientific community remains unconvinced. Many
(e.g., Kerr 2001, 1876) feel that more work needs to be done to exclude
the possibility of a nonbiotic origin for the prismatic magnetite. Another
concern is the lack of new evidence for the biotic hypothesis. If researchers
utilizing sophisticated analytical techniques had discovered additional
traces that were as difficult to explain nonbiotically as the prismatic mag-
netite, the case for fossilized Martian life would be greatly enhanced, just
as the discovery of shocked quartz (which followed the discovery of irid-
ium) in the K-T boundary was pivotal in convincing skeptics that the
Alvarez hypothesis was more plausible than its rivals. Given the overde-



)*+,$#*!"% - &./&#*0&',"% +!*&'!&+ 494

termination of the past by the localized present, one would expect such
traces to exist if the biotic hypothesis were true. Despite much effort with
very sophisticated equipment, however, they haven’t been found. Until
scientists are convinced that chemically pure, prismatic magnetite cannot
be produced by nonbiotic processes or have found additional traces that
are as difficult to explain in terms of nonbiotic processes, or, alternatively,
a unified, physically consistent, nonbiotic explanation is found for all the
perplexing features in ALH84001, the fossilized Martian life hypothesis
will remain viable but highly controversial. This is just what one would
expect of an historical hypothesis with suggestive but nonetheless ambig-
uous evidential support.

Just as the overdetermination of past events by localized present events
explains the practice of prototypical historical science, so the underdeter-
mination of future events by localized present events explains the practice
of classical experimental science. The hypotheses tested in classical exper-
imental science are generalizations (conjectured laws and theories), as op-
posed to statements about particular events. Nevertheless, like the traces
uncovered by historical research, the conditions manipulated in the lab
during classical experimental research are local phenomena. But they are
only partial causes (triggers) of what subsequently occurs. Accordingly,
there is a need to ferret out and control for additional factors that are
relevant to the total causal situation; otherwise, the ostensible confirma-
tions and disconfirmations of the target hypothesis may be mistaken. This
is why experimental scientists spend so much time systematically rejecting
auxiliary assumptions that they accepted in earlier experiments. They are
not trying to disprove their hypotheses or to save them from falsification.
They are trying to identify false positives and false negatives, which are
always a threat since localized events typically underdetermine their ef-
fects. In brief, classical experimental research is best interpreted as an
attempt to circumvent the inevitable underdetermination of experimental
results by test conditions.

5. Conclusion. There are fundamental methodological differences be-
tween prototypical historical science and classical experimental science
vis-à-vis the testing of hypotheses. These differences represent different
patterns of evidential reasoning—patterns that are designed to exploit
different sides of an asymmetry of epistemic overdetermination. Insofar
as they are concerned with identifying particular past causes of current
phenomena, historical researchers cannot directly test their hypotheses
by means of controlled experiments. They can, however, proliferate al-
ternative explanations for the traces they observe and then search for a
smoking gun to discriminate among them. The overdetermination of the
past by the localized present provides the rationale for this work, ensuring



!"#$% &. !%&%"'(495

that the probability of finding such traces is fairly high. In contrast, in
their efforts to secure trustworthy laboratory evidence for the generali-
zations that lie at the heart of their investigations, classical experimental-
ists face the underdetermination of the future by the localized present.
This explains why they spend so much time systematically controlling for
assumptions that they accepted in earlier experiments; they are trying to
eliminate false positives and false negatives. In other words, although ex-
perimentalists and historical researchers both ultimately infer causes from
effects, the evidential relations that they exploit are different and this dif-
ference reflects the fact that events are causally connected in time in an
asymmetric manner. It is important to keep in mind that nothing that I
have said contravenes the tentative nature of the support offered by em-
pirical evidence for a hypothesis, nor challenges the claim that many of
the factors that play a central role in the acceptance of a hypothesis are
psychological and political. But this is just as true of experimental science
as it is of historical science. The point is, although there are fundamental
methodological differences between historical research and classical ex-
perimental research, these differences do not support the contention that
prototypical historical science is epistemically inferior to classical experi-
mental science. Indeed, I suspect that the differences that I have identified
could be reconstructed as inferences in accordance with Bayes’ theorem.
Unfortunately, however, it is beyond the scope of this paper to provide a
theory of confirmation.

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