Full Idea
If the same consciousness can be transferred from one thinking substance to another, then two thinking substances may make but one person. For one consciousness being preserved, whether in the same or different substances, personal identity is preserved.
Gist of Idea
Identity must be in consciousness not substance, because it seems transferable
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.27.13)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.338
A Reaction
I take the sensible modern view to be that the transfer of the same consciousness between two different physical substances is absurd, since consciousness is (at the very least) entailed by the physical state. Could there be mentally identical twins?