What to read
Citation: John Locke. An Essay Concerning Human Understanding. 1690 (Edited by P. H. Nidditch, Oxford: Clarendon Press, 1975).
Read: book II, chapter 27 (“Of identity and diversity”), sections 9-10, 16-23.
Where to find it: there should be multiple copies of the Essay in just about any library. You can find this chapter, along with some 18th century criticisms by Bishop Butler and Thomas Reid, in a collection edited by John Perry titled Personal Identity. Finally, you can find just about everything Hobbes, Hume, and Locke wrote in the InteLex PastMasters series, a searchable database on the web [requires University of Chicago connection]. Look under “British philosophy 1600-1900.”
What to look for
The main point of interest is Locke’s attempt to define personal identity in terms of consciousness. ‘Consciousness extended backwards’ seems to be memory.
This has some strange implications. For example, I cannot forget any part of my past; if I cannot remember having had any experiences last Tuesday, then I am not identical with any person who was alive last Tuesday. That seems to mean that I did not exist last Tuesday, even if I did exist last Monday and last Wednesday (suppose that I can remember experiences on both days).
Compare Hume’s alternative suggestions about the role of memory in producing the belief in personal identity over time.
I should mention one oddity of Locke’s presentation: he distinguishes between “person” and “man.” A person is a thinking thing. A man is a living human animal. Locke believed that a person could switch from one man to another.
This and other details of Locke’s view are discussed at greater length on the website for my course on personal identity.