"What would you select as an example that could stand for any changeable thing such that your analysis of it would be applicable to any of them?

Just as the child's confused knowledge of adults employs 'father' and 'mother', so the analysis of things that come to be as the result of a change, physical objects, we proceed in terms of particular examples. But it is not what is particular or peculiar to them that is at issue, but what they convey that would be true of any other physical object."

Aristotle uses this: Man becomes musical.

"Any change involves a subject."

"In any change, the subject comes to acquire a characteristic it did not previously have, indeed lacked. But being deprived of a characteristic and having that characteristic are contraries.

Thus, on the basis of his own analysis Aristotle concludes that, at a minimum, any change involves a subject and contraries. This coincides with what he took to be the implication of the various views of his predecessors. It is what they and he are talking about that explains this."

-Excerpt from Introduction to Thomas Aquinas study materials, found at catholicthinkers.org