This is true not only for parts of the body but for the whole body as well, Locke insists.
If the consciousness of one man were somehow transferred into another body so that the second body now contained all the memories of thoughts and actions that the first man once contained (but does no more), the person would now inhabit the second body and not the first.
Of the three basic kinds of complex idea, relations are the easiest to understand.
The mind can consider any idea as it stands in relation to any other.
Instead, personal identity has only to do with consciousness: it is by the consciousness of one's present thoughts and actions that the self is conceived, and it is through the continuous link of memory that the self is extended back to past consciousness.
Locke's argument for this claim rests on his idea of identity, which is defined in terms of a comparison between something presently existing and the existence of that thing at an earlier time.
Still, there is no reason to assume, on this view, that consciousness cannot be transferred from one body or mind to another (think of a science fiction example where all of one's thoughts are transferred to a computer chip, so that consciousness moves from the mind to the computer).
That consciousness exists independent of material substance (i.e. Locke gives an example to illustrate just how intuitive this notion is: When a finger is cut off from a man's hand, it is clearly no longer a part of his consciousness; he is no more conscious of any effects on this finger than he is conscious of effects on any other man's finger.
It is Locke's third and final category of relational ideas, ideas of identity and diversity, that is of great importance to the history of philosophy. It is in the context of this discussion that Locke presents his theory of personal identity, that is, his theory of what makes us the same person over time.
According to Locke, remaining the same person has nothing to do with remaining the same substance, either physical or mental.