a metaphysical theory that objects have essences and that there is a distinction between essential and non-essential or accidental predications. Different issues have, however, been central in debates about essences and essential predication in different periods in the history of philosophy. In our own day, it is commitment to the notion of de re modality that is generally taken to render a theory essentialist; but in the essentialist tradition stemming from Aristotle, discussions of essence and essential predication focus on the distinction between what an object is and how it is. According to Aristotle, the universals that an ordinary object instantiates include some that mark it out as what it is and others that characterize it in some way but do not figure in an account of what it is. In the Categories, he tells us that while the former are said of the object, the latter are merely present in it; and in other writings, he distinguishes between what he calls kath hauto or per se predications (where these include the predication of what-universals) and kata sumbebekos or per accidens predications (where these include the predication of how-universals). He concedes that universals predicated of an object kath hauto are necessary to that object; but he construes the necessity here as derivative. It is because a universal marks out an entity, x, as what x is and hence underlies its being the thing that it is that the universal is necessarily predicated of x. The concept of definition is critically involved in Aristotle’s essentialism. First, it is the kind – infima species – under which an object falls or one of the items (genus or differentia) included in the definition of that kind that is predicated of the object kath hauto. But, second, Aristotle’s notion of an essence just is the notion of the ontological correlate of a definition. The term in his writings we translate as ‘essence’ is the expression to ti ein einai (the what it is to be). Typically, the expression is followed by a substantival expression in the dative case, so that the expressions denoting essences are phrases like ‘the what it is to be for a horse’ and ‘the what it is to be for an oak tree’; and Aristotle tells us that, for any kind, K, the what it is to be for a K just is that which we identify when we provide a complete and accurate definition of K. Now, Aristotle holds that there is definition only of universals; and this commits him to the view that there are no individual essences. Although he concedes that we can provide definitions of universals from any of his list of ten categories, he gives pride of place to the essences of universals from the category of substance. Substance-universals can be identified without reference to essences from other categories, but the essences of qualities, quantities, and other non-substances can be defined only by reference to the essences of substances. In his early writings, Aristotle took the familiar particulars of common sense (things like the individual man and horse of Categories V) to be the primary substances; and in these writings it is the essences we isolate by defining the kinds or species under which familiar particulars fall that are construed as the basic or paradigmatic essences. However, in later writings, where ordinary particulars are taken to be complexes of matter and form, it is the substantial forms of familiar particulars that are the primary substances, so their essences are the primary or basic essences; and a central theme in Aristotle’s most mature writings is the idea that the primary substances and their essences are necessarily one and the same in number.
The conception of essence as the ontological correlate of a definition – often called quiddity – persists throughout the medieval tradition; and in early modern philosophy, the idea that the identity of an object is constituted by what it is plays an important role in Continental rationalist thinkers. Indeed, in the writings of Leibniz, we find the most extreme version of traditional essentialism. Whereas Aristotle had held that essences are invariably general, Leibniz insisted that each individual has an essence peculiar to it. He called the essence associated with an entity its complete individual concept; and he maintained that the individual concept somehow entails all the properties exemplified by the relevant individual. Accordingly, Leibniz believed that an omniscient being could, for each possible world and each possible individual, infer from the individual concept of that individual the whole range of properties exemplified by that individual in that possible world. But, then, from the perspective of an omniscient being, all of the propositions identifying the properties the individual actually exhibits would express what Aristotle called kath hauto predications. Leibniz, of course, denied that our perspective is that of an omniscient being; we fail to grasp individual essences in their fullness, so from our perspective, the distinction between essential and accidental predications holds.
While classical rationalists espoused a thoroughgoing essentialism, the Aristotlelian conceptions of essence and definition were the repeated targets of attacks by classical British empiricists. Hobbes, e.g., found the notion of essence philosophically useless and insisted that definition merely displays the meanings conventionally associated with linguistic expressions. Locke, on the other hand, continued to speak of essences; but he distinguished between real and nominal essences. As he saw it, the familiar objects of common sense are collections of copresent sensible ideas to which we attach a single name like ‘man’ or ‘horse’. Identifying the ideas constitutive of the relevant collection gives us the nominal essence of a man or a horse. Locke did not deny that real essences might underlie such collections, but he insisted that it is nominal rather than real essences to which we have epistemic access. Hume, in turn, endorsed the idea that familiar objects are collections of sensible ideas, but rejected the idea of some underlying real essence to which we have no access; and he implicitly reinforced the Hobbesian critique of Aristotelian essences with his attack on the idea of de re necessities. So definition merely expresses the meanings we conventionally associate with words, and the only necessity associated with definition is linguistic or verbal necessity. From its origins, the twentieth-century analytic tradition endorsed the classical empiricist critique of essences and the Humean view that necessity is merely linguistic. Indeed, even the Humean concession that there is a special class of statements true in virtue of their meanings came into question in the forties and fifties, when philosophers like Quine argued that it is impossible to provide a noncircular criterion for distinguishing analytic and synthetic statements. So by the late 1950s, it had become the conventional wisdom of philosophers in the Anglo-American tradition that both the notion of a real essence and the derivative idea that some among the properties true of an object are essential to that object are philosophical dead ends. But over the past three decades, developments in the semantics of modal logic have called into question traditional empiricist skepticism about essence and modality and have given rise to a rebirth of essentialism. In the late fifties and early sixties, logicians (like Kripke, Hintikka, and Richard Montague) showed how formal techniques that have as their intuitive core the Leibnizian idea that necessity is truth in all possible worlds enable us to provide completeness proofs for a whole range of nonequivalent modal logics. Metaphysicians seized on the intuitions underlying these formal methods. They proposed that we take the picture of alternative possible worlds seriously and claimed that attributions of de dicto modality (necessity and possibility as they apply to propositions) can be understood to involve quantification over possible worlds. Thus, to say that a proposition, p, is necessary is to say that for every possible world, W, p is true in W; and to say that p is possible is to say that there is at least one possible world, W, such that p is true in W. These metaphysicians went on to claim that the framework of possible worlds enables us to make sense of de re modality. Whereas de dicto modality attaches to propositions taken as a whole, an ascription of de re modality identifies the modal status of an object’s exemplification of an attribute. Thus, we speak of Socrates as being necessarily or essentially rational, but only contingently snub-nosed. Intuitively, the essential properties of an object are those it could not have lacked; whereas its contingent properties are properties it exemplifies but could have failed to exemplify. The ‘friends of possible worlds’ insisted that we can make perfectly good sense of this intuitive distinction if we say that an object, x, exhibits a property, P, essentially just in case x exhibits P in the actual world and in every possible world in which x exists and that x exhibits P merely contingently just in case x exhibits P in the actual world, but there is at least one possible world, W, such that x exists in W and fails to exhibit P in W.
Not only have these neo-essentialists invoked the Leibnizian conception of alternative possible worlds in characterizing the de re modalities, many have endorsed Leibniz’s idea that each object has an individual essence or what is sometimes called a haecceity. As we have seen, the intuitive idea of an individual essence is the idea of a property an object exhibits essentially and that no other object could possibly exhibit; and contemporary essentialists have fleshed out this intuitive notion by saying that a property, P, is the haecceity or individual essence of an