Full Idea
Identity, when applied to persons, has no ambiguity, and admits of no degrees. It is the foundation of all rights and obligations, and of all accountableness.
Gist of Idea
Personal identity is the basis of all rights, obligations and responsibility
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.112
A Reaction
This seems to me to be one of the key mistakes in all of philosophy - thinking that items must always be all-or-nothing. If a person deteriorates through Alzheimer's, there seem to be obvious degrees of personhood. Responsibility comes in degrees, too.