Full Idea
A man challenges a thief in possession of his horse only on similarity. The testimony of witnesses to the identity of a person is commonly grounded on no other evidence. ...Evidence of our own identity is grounded in memory, and gives undoubted certainty.
Gist of Idea
The identity of a thief is only known by similarity, but memory gives certainty in our own case
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
With other people the best we can hope for is type-identity, hoping that each individual being is a unique type, but with otherselves we are always confident of establishing token identity. Could I have been someone different yesterday, without realising?