This dissertation retrieves the fundamental theology of Spanish (Basque) philosopher, Xavier Zubiri (1898–1983). Zubiri radically revises the responses of phenomenology and classical metaphysics to the persistent Western dualism between sense and understanding. Rather than focus upon the structure of consciousness or the intelligibility of being, he reduces things to the sheer, physical power of reality, and its actualization through human intellection. Providing a novel account of the human in terms of what he calls sentient intelligence, he also overcomes the modern dualism between nature and person. For Zubiri, the human uniquely searches and tests its conditioned freedom, i.e., its relative absoluteness. He calls this fundamental depth dimension of human reality "theologal," and identifies the absolutely absolute as "God."To present Zubiri's original systematics, which critically revises the categories of substance and causality, attention to his lexical apparatus is necessary—analyzing terms such as formality, fundamentality, problematicity, religation, respectivity, and substantivity. Also, the Spanish language uniquely makes possible a distinction in "being" through two words, ser and estar, which receives attention. Beyond linguistic usage, Zubiri exhibits distinctively Spanish characteristics of thought, which contemporary portraits of Western philosophy and theology have tended to overlook. In particular, he revises the Spanish brand of existentialism found in his teacher then colleague, José Ortega y Gasset. To show furthermore the purchase of Zubirian thought, two other critical comparisons are made—with John Zizioulas, an influential dialogue partner for Western theology, and his relational ontology; and with Maurice Blondel, a major inspiration behind the changes in conciliar era theology, and his actional account of the infinite.Crucially for theology, Zubiri provides a concrete and modern metaphysics of personal experience that is not at odds with reason. He identifies "experience" as a methodical mode of rational inquiry, such that experience is not reducible to sensation, cognition, or life; rather, it is the physical testing or probation of one's own articulation of what reality most profoundly is. Human reality is constitutively the problem of God, and the experience of such is rationally probative. Articulate identification of this problematicity is fundamental to all further theological investigation.