From my understanding, Aristotelian metaphysics uses the concepts of potentiality and actuality to explain change. Since being cannot come from non-being, when change occurs a potential is simply being actualized. However, what exactly is a potential, metaphysically speaking? What kind of substance is a potential, and what happens to it when it is actualized?
Potentiality is not a substance. Substances are matter+form composites.
As St. Thomas Aquinas writes in his short word On The Principles of Nature 5.,
everything which is in potency can be called matter
omne quod est in potentia potest dici materia
Matter is a mode of being midway between non-existence and substance.
Okay, so when a change occurs, what exactly is the thing that is changing? The reason why I thought potentiality was a thing is because I thought that's how Aristotle's was trying to solve the problem of change. Since I thought potentiality was a thing, it would solve the problem of being coming from non-being. Mar 23, 2019 at 1:10
A substantial change (e.g., Na + Cl → NaCl, a frog becoming a human, etc.) involves two substances, the first of which is "corrupted" (reduced to primary matter) and the second of which is generated (primary matter informed with a substantial form); see the diagram here, this answer, this chapter on act & potency, or Aristotelian Definition of Motion [=Change].– GeremiaMar 23, 2019 at 1:29