Haecceity (/hɛkˈsiːɪti, hiːk-/; from the Latin haecceitas, which translates as “thisness”) is a term from medieval scholastic philosophy, first coined by Duns Scotus, which denotes the discrete qualities, properties or characteristics of a thing which make it a particular thing. Haecceity is a person or object’s “thisness”, the individualising difference between the concept ‘a man’ and the concept ‘Socrates’ (a specific person). Haecceity is a literal translation of the equivalent term in Aristotle’s Greek to ti esti (τὸ τί ἐστι) or “the what it is.”
Charles Sanders Peirce later used the term as a non-descriptive reference to an individual.
Haecceity and quiddity
Haecceity may be defined in some dictionaries as simply the “essence” of a thing, or as a simple synonym for quiddity or hypokeimenon. However, such a definition deprives the term of its subtle distinctiveness and utility. Whereas haecceity refers to aspects of a thing which make it a particular thing, quiddity refers to the universal qualities of a thing, its “whatness”, or the aspects of a thing which it may share with other things and by which it may form part of a genus of things.