Full Idea
If the same consciousness can be transferred from one intelligent being to another, then two or twenty beings may be the same person. If he may lose the consciousness of actions done by him, one intelligent being may be two or twenty different persons.
Gist of Idea
If consciousness is transferable 20 persons can be 1; forgetting implies 1 can be 20
Source
Thomas Reid (Essays on Intellectual Powers 3: Memory [1785], III.Ch 6)
Book Reference
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.114
A Reaction
Reid says Locke was aware of these two implications of his theory of personal identity (based on consciousness). The first example is me replicated like software. The second is if I forget that I turned the light off, then who did turn the light off?