Reid has lots of objections to Locke's view that personal identity is just made up of how far our conscious memory, or autobiographical memory, can extend.
First Objection: Reid thinks that when we say consciousness, we usually mean memory. But in philosophical discussions, we should distinguish between the two. Specifically, consciousness is always in the present and memory is of the past. Locke responds: one can remember something without having consciousness of that memory. We require that one does have conscious awareness of the memories that define one’s personal identity. Personal identity requires this experiential memory, which is just remembering things from a first-person perspective in a way that preserves the qualitative character of our experiences. We call this autobiographical memory.
Second Objection: Identity requires that something stays the same over time. Consciousness is in constant flux. Therefore, if our personal identity is the same as our consciousness, then our personal identity would go in and out of existence all the time. This seems silly. Locke responds: First, consciousness is not in total flux. We are still conscious when we sleep, perhaps. Second, personal identity is not limited by actual consciousness but our ability to be conscious of past memories. We do not need to be actively conscious of our memories in order to be the same person as who we were when we first experienced those memories.
Fourth Objection: Violation of the Transitivity of Identity. Imagine a brave officer. As a boy in school, he was beaten. He was successful in his first battle and later in life was a general. As an officer during his successful battle, he remembered the flogging. But as a general, he could remember being successful in his first battle but cannot remember being flogged as a boy. According to Locke, the old general would not be identical to the boy who was flogged. But this violates the transitivity of identity ( A = B, B = C so A = C). Response: Perhaps not just memory but also character traits constitute personal identity. Maybe not just memory but also personality can make someone numerically identical with himself or herself. Also, perhaps this indirect connection that is described in the case is sufficient for personal identity. In other words, the connections can be transitive.
Michael Jackson version of the brave officer: When Michael Jackson (MJ) released Thriller, he remembered being a member of the Jackson 5. Imagine that when he was on his deathbed, he could not remember being in the Jackson 5. According to Locke's theory, Thriller MJ is identical to Jackson 5 MJ. Also according to Locke's theory, Thriller MJ is identical to almost dead MJ. By the transitivity of identity, this means that Jackson 5 MJ is the same as almost dead MJ. However, according to Locke's theory, almost dead MJ cannot be identical to Jackson 5 MJ because almost dead MJ does not remember being in the Jackson 5. This violation of the transitivity of identity is a problem for Locke's theory