Substances, in a philosophical system, are those things which, according to that system, are the foundational or fundamental entities of reality (Stanford Encyclopedia of Philosophy). Eg., An Atomist considers atoms as the substance for they are the basic things from which everything is constructed, Philosopher David Hume considered Impressions and Ideas as substance, Philosopher Plato considered ‘Forms” as substance.

John Locke considered soul, body and a thinking element (the third term) as Substance. He considered Personal Identity to be founded on consciousness and not on substance of either soul or body, ie., Personal Identity consists not in the identity of the substance, but in the identity of consciousness. To paraphrase an example from Wikipedia, One may claim to be a reincarnation of Plato, therefore having the same soul substance. However, one would be the same person as Plato only if one had the same consciousness of Plato’s thoughts and actions that he himself did. Therefore, Personal Identity is not based on soul substance.

As I understand it, Locke holds that consciousness can be transferred from one soul to another and that Personal Identity goes with consciousness, ie., consciousness can be transferred from one substance to another, and thus, while the substance is changed, consciousness remains the same, thereby preserving the Personal Identity through the change.

From a personal perspective, what I think, feel do, experience, make up my Personal Identity. I am still the person I was, say, 10 years ago. But my body, mind and thought processes may have changed over time. These changes of substance have enhanced my consciousness. My Personal Identity remains the same, but it has been affected by these changes. So I agree with Locke’s point of view.