Full Idea
The identity of a person is a perfect identity: wherever it is real, it admits of no degrees; and it is impossible that a person should be in part the same, and in part different; because a person is a 'monad', and is not divisible into parts.
Gist of Idea
A person is a unity, and doesn't come in degrees
Source
Thomas Reid (Essays on Intellectual Powers 3: Memory [1785], III.Ch 4)
Book Reference
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.111
A Reaction
I don't accept this, because I don't accept the metaphysics needed to underpin it. To watch a person with Alzheimer's disease fade out of existence before they die seems sufficient counter-evidence. I believe in personal identity, but it isn't 'perfect'.