There is a long-standing puzzle about how Aristotle come to believe that the soul is the form of a body which possesses life potentially. This is because the hylomorphic theory of nature that Aristotle develops in his Physics to account for natural things and their changes in terms of material, formal, efficient, and final causes does not on its own provide a clear answer as to what the soul is. I argue that the way Aristotle tries to determine the answer to this question essentially involves an investigation into earlier Greek psychological theories and constrains him to adopt five theses that characterise hylomorphic psychology: the Hylomorphic Thesis that soul is a fulfilment of a bodily potential, the Efficient-Final Causal Thesis that the soul is an efficient and final cause of the body's motions, the Non-Uniformity Thesis that there is no one uniform kind of thing in nature called 'soul', the Part-Hood Thesis that the soul has capacity-parts, which are not spatially divisible from one another in a given body, and the Separability Thesis that the mental capacity of soul might exist separate from the body.