Full Idea
It being the same consciousness that makes a man himself to himself, personal identity depends on that only, whether it be annexed solely to one individual substance, or can be continued in a succession of several substances.
Gist of Idea
Our personal identity must depend on something we are aware of, namely consciousness
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.27.10)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.336
A Reaction
The counterexample would be a highly sophisticated robot that lacked consciousness. IF it could achieve 'sophisticated' behaviour, we might need personal identity to explain its utterances and doings.