That for the sake of which a substance changes is a reason for the substance being the kind of thing it is. For example, both Socrates and Plato are human beings, but Plato is not Socrates. Though they agree in their form, they differ in their matter. We must be careful to recognize that the material cause is understood as the specific type of matter of which a thing is made, so that bronze is the material cause of a statue, etc. Forms are universals, characteristics shared by many individuals.
Human beings are by definition one and all rational.
One can explain human behavior by appealing to humans' rationality. Socrates drank the hemlock because he believed it was the best thing to do, that is, insofar as he was exercising his rationality. To gain knowledge of the universal, we begin with perception of many individuals of a certain sort. Then in a movement called induction, we understand what is common among things of that sort. For example, we might determine by induction that a light's being near and its not twinkling are universally associated.