haecceity (from Latin haec, ‘this’), (1) loosely, thisness; more specifically, an irreducible category of being, the fundamental actuality of an existent entity; or (2) an individual essence, a property an object has necessarily, without which it would not be or would cease to exist as the individual it is, and which, necessarily, no other object has. There are in the history of philosophy two distinct concepts of haecceity. The idea originated with the work of the thirteenthcentury philosopher Duns Scotus, and was discussed in the same period by Aquinas, as a positive perfection that serves as a primitive existence and individuation principle for concrete existents. In the seventeenth century Leibniz transformed the concept of haecceity, which Duns Scotus had explicitly denied to be a form or universal, into the notion of an individual essence, a distinctive nature or set of necessary characteristics uniquely identifying it under the principle of the identity of indiscernibles. Duns Scotus’s haecceitas applies only to the being of contingently existent entities in the actual world, but Leibniz extends the principle to individuate particular things not only through the changes they may undergo in the actual world, but in any alternative logically possible world. Leibniz admitted as a consequence the controversial thesis that every object by virtue of its haecceity has each of its properties essentially or necessarily, so that only the counterparts of individuals can inhabit distinct logically possible worlds. A further corollary – since the possession of particular parts in a particular arrangement is also a property and hence involved in the individual essence of any complex object – is the doctrine of mereological essentialism: every composite is necessarily constituted by a particular configuration of particular proper parts, and loses its self-identity if any parts are removed or replaced. See also DUNS SCOTUS, ESSENTIAL – ISM , IDENTITY OF INDISCERNIBLES , META – PHYSIC. D.J.