¹ See Locke, John, An Essay Concerning Human Understanding, ed. Nidditch, P. H. (Oxford: Clarendon Press, 1975), III, III, 15Google Scholar.

² See, for example, Aristotle, Metaphysics, Z, 4–5.

³ See my The Possibility of Metaphysics: Substance, Identity, and Time (Oxford: Clarendon Press, 1998), ch. 1.

⁴ For more on the distinction between real or metaphysical possibility and ‘mere’ logical possibility, see again my The Possibility of Metaphysics, ch. 1.

⁵ See Kripke, Saul A., Naming and Necessity (Oxford: Blackwell, 1980)Google Scholar.

⁶ See Kripke, Saul A., ‘Identity and Necessity’, in Munitz, M. K. (ed.), Identity and Individuation (New York: New York University Press, 1971), pp. 135–64Google Scholar. I set aside, for present purposes, the serious doubts that may be raised concerning the cogency of this supposed proof, for which see further my ‘On the Alleged Necessity of True Identity Statements’, Mind 91 (1982), pp. 579–84 and my ‘Identity, Vagueness, and Modality’, in Bermúdez, J. L. (ed.), Thought, Reference, and Experience: Themes from the Philosophy of Gareth Evans (Oxford: Clarendon Press, 2005), pp. 290–310CrossRefGoogle Scholar.

⁷ For similar doubts, whose cogency seem to have been forgotten by many, see Salmon, Nathan U., Reference and Essence (Oxford: Blackwell, 1982)Google Scholar.

⁸ For further discussion of this general point, see my The Possibility of Metaphysics, ch. 1.

⁹ I am grateful to Paul Noordhof and Timothy Williamson for pressing me on this point in discussion.

¹⁰ It might perhaps be supposed that they would feel twice as heavy as one such object normally would. But how could any purely empirical consideration entitle one to suppose this? And even if they would feel twice as heavy as one such object normally would, how could mere observation enable us to distinguish this hypothetical situation from one in which a single object of the kind in question had twice the weight of a normal object of that kind? I should stress that in raising queries of this type, I have not the slightest intention to give succour to scepticism regarding our perceptual capacities, but only to bring to our attention the ways in which our use of those capacities to acquire empirical knowledge relies upon our grasp of certain a priori modal truths, in confirmation of my general thesis that knowledge of what is actual cannot be delivered by unaided experience—unaided, that is, by properly modal knowledge.

¹¹ See further my Kinds of Being: A Study of Individuation, Identity and the Logic of Sortal Terms (Oxford: Blackwell, 1989), ch. 2.

¹² Of course, there may be—indeed, are—metaphysians who, for their own reasons, dispute the truth of the proposition that two different material objects of the same kind cannot coincide: but since their reasoning, for what it is worth, is itself based at least in part on modal considerations, they pose no threat to the more general claim that I am defending, namely, that all empirical judgements rest on modal presuppositions. They pose only a threat to the suggestion that, in the case of Hesperus and Phosphorus, the modal presupposition that I have cited is a correct one. But I should emphasize that I myself think that the presupposition in question is a metaphysically sound one, agreeing in this respect with Oderberg, David S.: see his ‘Coincidence Under a Sortal’, The Philosophical Review 105 (1996), pp. 145–71CrossRefGoogle Scholar.

¹³ Jackson, Frank, for instance, urges precisely this in his recent book, From Metaphysics to Ethics: A Defence of Conceptual Analysis (Oxford: Clarendon Press, 1998): see p. 52Google Scholar. The expression ‘watery stuff’, thus understood, is borrowed from Chalmers, David J., The Conscious Mind (Oxford: Oxford University Press, 1996), p. 57Google Scholar.

¹⁴ In fact, ‘Water is the watery stuff (around here)’ and ‘The watery stuff (around here) is H₂O’ are both supposed to be contingent truths (the first a priori and the second a posteriori), in which case we really do have a problem on our hands, because a necessary truth—which is what ‘Water is H₂O’ is supposed to be—cannot be deduced from purely contingent premises. If we take the definite description ‘the watery stuff (around here)’ in the second premise to be an implicitly rigidified one, that problem goes away but is replaced by another: for this description in the first premise must not be taken to be an implicitly rigidified one if the first premise is to be interpreted as an example of the contingent a priori.

¹⁵ See further Jackson, From Metaphysics to Ethics, ch. 2 and ch. 3, where the history of this approach is documented and the approach itself is endorsed.

¹⁶ The view that all laws of nature are in fact metaphysically necessary has been advanced by some philosophers recently: see, for example, Ellis, Brian, Scientific Essentialism (Cambridge: Cambridge University Press, 2001)Google Scholar. However, this is still very much a minority view and it is one that I myself challenge elsewhere: see my The Four-Category Ontology: A Metaphysical Foundation for Natural Science (Oxford: Clarendon Press, 2006), ch. 9 and ch. 10.

¹⁷ For extensive recent discussion of the relationship between conceivability and possibility, see Gendler, T. S. and Hawthorne, J. (eds), Conceivability and Possibility (Oxford: Clarendon Press, 2002)Google Scholar.

¹⁸ See again Locke, An Essay Concerning Human Understanding, III, III, 15.

¹⁹ The historical source of this view lies, of course, with Aristotle, whose phrase τo τι ην ɛιναι is standardly translated as ‘essence’: see Aristotle, Metaphysics Z, 4. Its more literal meaning is ‘the what it is to be’ or ‘the what it would be to be’.

²⁰ I do not attempt to offer here a semantic analysis of expressions such as ‘what X is’, ‘what it is to be X’ or ‘the identity of X’, although that is no doubt an exercise that should be undertaken at some stage in a full account of what I am calling serious essentialism. I assume that our practical grasp of the meaning of such expressions is adequate for a preliminary presentation of the approach of the sort that I am now engaged in.

²¹ For my own account of what ontological categories we should recognize and which we should regard as fundamental, see my The Four-Category Ontology, especially Part I.

²² Note that I ask only how we can talk or think comprehendingly about a thing if we do not know what it is—not how we can perceive a thing if we do not know what it is. I am happy to allow that a subject S may, for example, see an object O even though S does not know what O is. Seeing, however, is not a purely intellective act. Indeed, of course, even lower animals that cannot at all plausibly be said to understand what objects exist in their environment, may nonetheless be said to see or feel or smell some of those objects.

²³ Perhaps, indeed, all I need to know about cats is that they are animals or living organisms and perhaps, likewise, all I need to know about Tom is which animal or living organism he is.

²⁴ Of course, it is fashionable at present to suppose that our talk and thought have, in general, their referents in the ‘external’ world secured through the existence of appropriate causal links between certain constituents of our talk and thought—certain of our linguistic and mental ‘representations’—and various extra-linguistic and extra-mental entities belonging to that world: links that can, and mostly do, obtain without our needing to have any knowledge of them. On this sort of view, it may be supposed, my talk and thought can fasten upon Tom because there is an appropriate causal link between the name ‘Tom’, as I have learnt to use it, and Tom—and an analogous causal link between a certain ‘mental representation’ of mine (perhaps a certain ‘symbol’ in the putative ‘language of thought’ supposedly utilized by my brain) and Tom. I will only say here that I cannot begin to understand how it might seriously be supposed that a linkage of this sort could genuinely suffice to enable me to talk and think comprehendingly about Tom, even if it is conceded that there is a (relatively anodyne) notion of ‘reference’ that could perhaps be satisfactorily accounted for by a causal theory of the foregoing sort. I should emphasize, then, that I am not presently concerned to challenge the so-called causal theory of reference, much less to defend in opposition to it some sort of neo-Fregean theory of reference as being mediated by ‘sense’. Rather, I am simply not interested, at present, in semantic questions or rival semantic theories, but rather in the purely metaphysical question of how it is possible to be acquainted with an object of thought: my answer being that it is so through, and only through, a grasp of that object's essence—that is, through knowing what it is.

²⁵ There are, broadly speaking, two different views of what a set is: one which takes a set simply to be the result of—as David Lewis puts it—‘collecting many into one’, and another which takes a set to be the extension of a property or of a concept. For Lewis's, remark, see his Parts of Classes (Oxford: Blackwell, 1991), p. viiGoogle Scholar. I see no compelling reason why, in principle, our ontology should not accommodate sets in both of these understandings of what they are. But since I am using the example of sets only for illustrative purposes, this is a matter on which I can afford to remain agnostic here.

²⁶ See further my The Possibility of Metaphysics, ch. 6, or alternatively my ‘Ontological Dependence’, The Stanford Encyclopedia of Philosophy (2005), ed. E. N. Zalta, http://plato.stanford.edu.

²⁷ Thus, at one point Locke remarks: ‘[W]e come to have the Ideas of particular sorts of Substances, by collecting such Combinations of simple Ideas, as are by Experience ... taken notice of to exist together, and are therefore supposed to flow from the particular internal Constitution, or unknown Essence of that Substance’ (Essay, II, XXIII, 3).

²⁸ See, especially, Kripke, , Naming and Necessity and Hilary Putnam, ‘The Meaning of “Meaning”’, in his Mind, Language and Reality: Philosophical Papers Volume 2 (Cambridge: Cambridge University Press, 1975)Google Scholar.

²⁹ Note here that it is vital not to confuse the following two forms of assertion: ‘It is part of the essence of X that X has property P’ and ‘Property P is part of the essence of X’. Far from it being the case that these two forms of assertion are equivalent, only the first of them is acceptable according to my conception of essence. For, on the assumption that a property, P, is an entity, the second implies that the essence of X has an entity as a part, and consequently that the essence of X is itself an entity, contrary to my principle that essences are not entities. By contrast, the first form of assertion has no such unwanted implication.

³⁰ See, for example, Quine, W. V., ‘Existence and Quantification’, in his Ontological Relativity and Other Essays (New York: Columbia University Press, 1969)Google Scholar.

³¹ I am reassured by finding an independent expression of support for this view in the Introduction to Fine's, Kit recent book, Modality and Tense (Oxford: Clarendon Press, 2005): see p. 11CrossRefGoogle Scholar.

³² Notoriously, Descartes is supposed to have claimed, in the Second Meditation, to know that he existed before he knew what he was—that is, before he grasped his own essence. But it seems to me that any such claim must be construed as being either disingenuous or else intended non-literally, if it is not to be dismissed as being simply incomprehensible. It might, for instance, be taken to imply merely that Descartes was certain that the word ‘I’ had a reference, before knowing what that reference was. To be accurate, though, what Descartes actually says is ‘But I do not yet have a sufficient understanding of what this “I” is, that now necessarily exists’: see Descartes, René, Meditations on First Philosophy, trans. Cottingham, J. (Cambridge: Cambridge University Press, 1986), p. 17Google Scholar. That is consistent with saying that Descartes does already grasp his own essence, but needs to clear his mind of confused thoughts concerning it. Query: might we not come to know what X is neither before nor after discovering that X exists, but simultaneously with that discovery? Well, I see no reason to deny this possibility in some cases. But that concession need not be taken to undermine the claim that, in general, we can know the essence of something X before knowing whether or not X exists.

³³ The extent to which the Kripke–Putnam doctrine has become a commonplace of contemporary analytic philosophy is illustrated by the following remark of Frank Jackson's, which he makes simply in passing and without acknowledging any need to justify it: ‘[W]e rarely know the essence of the things our words denote (indeed, if Kripke is right about the necessity of origin, we do not know our own essences)’: see his From Metaphysics to Ethics, p. 50. Yet, I would urge, it should strike one as being odd to the point of paradoxicality to maintain that we can talk or think comprehendingly about things without knowing what it is that we are talking or thinking about—that is, without grasping their essences. The charitable conclusion to draw would be that philosophers like Jackson do not use the term ‘essence’ in what Locke called its ‘proper original signification’. Now, of course, Locke himself says that the ‘real’ essences of material substances are unknown to us—and the Kripke–Putnam doctrine is recognizably a descendent of Locke's view, to the extent that it identifies the ‘real essences’ of material substances with their ‘internal constitutions’, many of which are certainly still unknown to us and may forever continue to be so. But Locke, at least, concluded—unlike modern adherents of the Kripke–Putnam doctrine—that ‘the supposition of Essences, that cannot be known; and the making them nevertheless to be that, which distinguishes the Species of Things, is so wholly useless ... [as] to make us lay it by’ (Essay, III, III, 17) and he accordingly appeals instead to what he calls nominal essences. The correct position, I suggest, is neither Locke's nor that of the Kripke–Putnam doctrine, but rather (what I take to be) Aristotle's: that the real essences of material substances are known to those who talk or think comprehendingly about such substances—and consequently that such essences are not to be identified with anything that is not generally known to such speakers and thinkers, such as the ‘particular internal constitution’ of a material substance, or a human being's (or other living creature's) ‘origin’ in the Kripkean sense.

³⁴ For extended discussion of the need to distinguish between these two species of possibility, see my The Four-Category Ontology, ch. 9 and ch. 10.

³⁵ I say ‘alleged’ intuitions because I suspect that many who say they have them do so only because it has become part of current philosophical orthodoxy to assume that ‘we’ do.

³⁶ Compare Fine, Kit, ‘Essence and Modality’, in Tomberlin, James E. (ed.), Philosophical Perspectives, 8: Logic and Language (Atascadero, CA: Ridgeview, 1994)Google Scholar.

³⁷ Note that analogously, then, it could be conceded that H₂O molecules necessarily compose water without it being conceded that it is part of the essence of water to be composed of H₂O molecules—for the necessity could be explained instead as arising from the essence of H₂O molecules.

³⁸ See further my ‘Substantial Change and Spatiotemporal Coincidence’, Ratio 16 (2003), pp. 140–60, and my ‘Material Coincidence and the Cinematographic Fallacy: A Response to Olson’, The Philosophical Quarterly 52 (2002), pp. 369–72, the latter being a reply to Olson, Eric T., ‘Material Coincidence and the Indiscernibility Problem’, The Philosophical Quarterly 51 (2001), pp. 337–55CrossRefGoogle Scholar.