Forms are a big deal in Aristotelian philosophy, and hence in Thomist philosophy. That which is per se to a being, that which makes it what it is, is its substantial form. Thus, your dog has the substantial form of a dog. There is also accidental form, such as a being’s color. Your dog might be brown, but it can be dyed blue, thus changing its form.

Note that form is not shape. A chair has the form of chairness, for example. But think about all the many different kinds of chair you’ve seen, all of which have the form of chairness, but which have many, many different shapes. If a being has an accidental form, Thomas refers to it as having the form per accidens rather than per se.

Most (all?) change involves gaining or losing a form. The terms generation, corruption, and alteration, which Thomas uses in CT 4, refer specific kinds of change. Generation is the gaining of a new substantial form. If I have an axe head, and an axe handle, and I put them together, I’ve made an axe. The two pieces together are a new being, which has a new substantial form, that of being an axe. Corruption is the loss of substantial form. If I hit you in the head with the axe, your body loses its substantial form, that of being a rational animal; it is merely a dead body. Alteration is the gain or loss of accidental form. If you dye your dog blue, you now have a blue dog: your dog remains a dog, though perhaps a disgruntled one.

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