quidditas

Quidditas (Latin for "whatness") is a philosophical term that denotes the essential nature of a thing—the set of properties that define its kind and without which the thing would not be what it is. It refers to the intrinsic essence that makes a thing the kind of thing it is, rather than features that may vary without altering that identity.

In medieval scholasticism, quidditas functions as the form or essential nature of a substance, the principle that grounds a thing’s being. It is contrasted with haecceitas, or the "thisness" that individuates a specific object, and with accidental properties that can change without altering the underlying kind. Thus, the quidditas is closely linked to Aristotelian form and essence.

The concept is used to articulate what properties are essential to a kind as opposed to contingent or incidental features. It informs debates on the relationship between essence and existence, necessary versus contingent properties, and the intelligibility of natural kinds.

In contemporary philosophy, the Latin term is less common, but the underlying idea persists in discussions of essence and essential properties, and in essentialist accounts of kinds and natural classifications. Quidditas remains a traditional way of naming the core attributes that define what a thing fundamentally is, beyond what it happens to be at any given moment.

See also: essence, form, substance, essentialism, haecceity, Aristotelianism.