Full Idea
Socrates can be assigned a haecceity: an essential property of 'being Socrates' which (unlike the property of 'being identical with Socrates') may be regarded as what 'makes' its possessor Socrates in a non-trivial sense, but is simple and unanalysable.
Clarification
Pronounced 'hex-ey-ety'
Gist of Idea
A haecceity is the essential, simple, unanalysable property of being-this-thing
Source
Penelope Mackie (How Things Might Have Been [2006], 2.2)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.21
A Reaction
I don't accept that there is any such property as 'being Socrates' (or even 'being identical with Socrates'), except as empty locutions or logical devices. A haecceity seems to be the 'ultimate subject of predication', with no predicates of its own.