John Locke's criterion for personal identity over time is that there must be consciousness, or a persisting memory, to link the past with the present. A person, therefore, according to Locke, is

a thinking intelligent Being, that has reason and reflection, and can consider itself as itself, the same thinking thing in different times and places.

Locke proposes a thought experiment in which the soul of a prince enters the body of a cobbler. The prince's consciousness and memories are part of his soul, and therefore the person in this new hybrid creation is that of the prince, whereas the man is that of the cobbler. Since the person usually persists inside the man, Locke says that we do not usually trouble to make this distinction, but when the two are separated, it quickly becomes clear that the person is the consciousness, whereas the man is the physical human being.

David Hume, by contrast, refused to admit that there was such a thing as a unified and coherent self or person. Hume said that what people think they experience as personal identity and consciousness is in fact

nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement.

It is the strength of these perceptions and the rapidity with which they occur that, according to Hume, creates the illusion that a single, consistent person is experiencing them. This belief is natural, but according to Hume, it is not rooted in logic, and when you try to examine the person doing the perceiving, they immediately disappear.