  Reid interprets Locke as holding what is now called the Memory Theory of personal identity (Essays, 277). On this theory, personal identity consists in memory; sameness of episodic memory is metaphysically necessary and sufficient for sameness of persons. In other words, on the Memory Theory, what makes a person identical with herself over time is her remembering or being able to remember the events to which she was witness or agent. If she cannot episodically remember an event, then she is not identical with any of the persons who was witness or agent to the event. In such a case, she would bear the same relation to that event as any other person for whom a memory of the event could rise at best to the level of a semantic memory. If she can episodically remember an event, then her recollection or ability to recall that event makes her identical with the person represented in that memory as agent or witness to the event.