Haecceity

Haecceity (/hɛkˈsiːɪti, hiːk-/; from the Latin haecceitas, which translates as "thisness") is a term from medieval scholastic philosophy, first coined by followers of Duns Scotus to denote a concept that he seems to have originated: the discrete qualities, properties or characteristics of a thing that make it a particular thing. Haecceity is a person's or object's thisness, the individualising difference between the concept "a man" and the concept "Socrates" (i.e., 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 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 that make it a particular thing, quiddity refers to the universal qualities of a thing, its "whatness", or the aspects of a thing it may share with other things and by which it may form part of a genus of things.

Duns Scotus makes the following distinction:

Because there is among beings something indivisible into subjective parts—that is, such that it is formally incompatible for it to be divided into several parts each of which is it—the question is not what it is by which such a division is formally incompatible with it (because it is formally incompatible by incompatibility), but rather what it is by which, as by a proximate and intrinsic foundation, this incompatibility is in it. Therefore, the sense of the questions on this topic [viz. of individuation] is: What is it in [e.g.] this stone, by which as by a proximate foundation it is absolutely incompatible with the stone for it to be divided into several parts each of which is this stone, the kind of division that is proper to a universal whole as divided into its subjective parts?

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