Aristotle asks: how is change possible?
For context, the pre-Socratic philosopher Parmenides thought that nothing can come from nothing, that what is cannot come or cease to be, and that change is therefore impossible. Change is merely an illusion, he thought, and the sum of reality, or even every aspect of reality, is an unchanging whole. Others contended that at all times everything changes in every way.
Aristotle agrees with Parmenides in his two assumptions, but instead concludes that there must be something permanent in all change; all change, in other words, is in a subject. For example:
1) The unmusical becomes the musical (Plato) must be false, since something is coming from a lack of that something
Where does this change in the subject come from? According to Aristotle, from his potential for musicality and from his interaction with other objects and subjects; but Socrates takes nothing from the world when observing and learning from rhythm, harmony etc. and so the change is all in the subject, in the conversion from potentiality to actuality.
But given that Aristotle rejected the notion that everything changes in every way, what underlies change when a subject comes into existence? If we are composed of matter and form (and are therefore substantial complexes, e.g. a house is composed of bricks, and the particular arrangements of those bricks), then what changes: matter, form, or both? Aristotle thought that it was the form that changed, whilst the matter unerlies the change. The matter that Socreates' body was composed of didn't change, but its form did. The same applies to the bronze.
This is a logical distinction useful in understanding the nature of substance. It holds that things (subjects & objects) are not identical to the matter that they are made up of:
1) My body's cells will die and be replaced as time passes
Change is the actuality of the potential qua such, i.e. the actualisation of a potential within a substance whilst aspects of that substance's matter (and form) persist
Objection: quantum physics again makes some of Aristotle's ideas questionable. Changes at the quantum level and in the zero point energy field suggest that matter can come in and out of existence from nothing, and without direction. The quantum level seems so random that it disobeys all of his laws of change. This either means that we can allow for two sets of rules, or that one set is wrong; either change at the quantum level does obey Aristotle's laws, and we have yet to see evidence, or at the super-quantum level all change is random.