Term used by Duns Scotus for that in virtue of which an individual is the individual that it is: its individuating essence making it this object or person. Haecceitism is the doctrine that there are such individual concepts. In modern discussion the term was revived by the American philosopher of language David Kaplan (‘How to Russell a Frege-Church’, Journal of Philosophy, 1975) in the idiom of possible worlds : ‘the doctrine that holds that it does make sense to ask—without reference to common attributes or behaviour—whether this is the same individual in another possible world…and that a common “thisness” may underlie extreme dissimilarity or distinct thisnesses may underlie great resemblance.’ See counterpart theory.
Philosophy dictionary. Academic. 2011.