Form is not shape or surface appearance but the intelligible organising principle that makes a thing what it is. In living beings, the soul is the form of the body — not a separate substance imprisoned in flesh but the very principle that makes the body a living body rather than a corpse. The soul of a plant makes it grow and nourish itself; the soul of an animal adds sensation and locomotion; the human soul adds rational activity. To ask what the soul is, for Aquinas, is to ask what principle of actuality organises the human body into a living, sentient, rational being.
If form is what makes a thing a certain kind, matter is what makes it this particular individual of that kind. Two human beings share the same form — the same specific essence — but they are numerically distinct because their common form is received in different parcels of matter. Matter is thus the ground of numerical multiplicity within a species. This has important consequences: angels, as purely formal beings, cannot be many within a single species — each angel is its own species, differing in kind and not merely in matter from every other. Human individuation, grounded in matter, means that each human soul is truly individual even as it shares a common rational nature with all others.
The hylomorphic account also explains change: in accidental change, form changes while the substance persists; in substantial change, matter takes on a new substantial form, and one kind of thing becomes another. Death, for instance, is the separation of soul from body: the body's organisation by a living principle ends, and the material substrate is reorganised according to the forms of the elements. Aquinas holds that the human soul, as a subsistent form capable of intellectual operation, persists after bodily death — not as a complete substance but awaiting the resurrection in which it will be reunited with a glorified body.
Hylomorphism is introduced in On Being and Essence, Chapter II, and developed throughout the Summa Theologiae, especially in the Treatise on Man (I QQ.75–102). The doctrine derives from Aristotle's Physics and Metaphysics, as transmitted and transformed by Arabic commentary.
