Locke's “wonderful mistake,” in pinning down what memory is

 Butler accuses Locke of a “wonderful mistake,” which is that he failed to recognize that the relation of consciousness presupposes identity, and thus cannot constitute it (Butler 1736, 100). 

In other words, I can remember only my own experiences, but it is not my memory of an experience that makes it mine; rather, I remember it only because it's already mine. 

So while memory can reveal my identity with some past experiencer, it does not make that experiencer me. What I am remembering, insists Butler, are the experiences of a substance, namely, the same substance that constitutes me now.