My dissertation provides a novel interpretation of Aristotle’s conception of the foundational notion of ‘parts of soul’ (moria psuchēs) in his De Anima. Interpreters have failed to explain how Aristotle can maintain two philosophically attractive and textually grounded claims: that there are multiple discrete parts of soul, and that soul is the unified and unifying form (eidos) of the living organism. Contrary to most interpreters, I argue that both claims are genuinely endorsed by Aristotle and, when correctly understood, compatible and crucial to the project of De Anima as a whole. First, psychic parts are not spatial parts, but are instead the definitionally basic or primitive capacities of an organism (nutrition, perception, intellect); these basic capacities, in turn, provide the explanatory foundation for understanding all other vital capacities. Second, Aristotle formulates a sophisticated account of psychic unity, according to which psychic parts are ‘present potentially’ within the soul: the soul, then, is no mere aggregate of parts, but is actually and essentially a whole, even while having parts.