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necessity of water's being H O is not available a priori is that though what is conceptually possible and impossible is
2
available in principle to reason alone given sufficient grasp of the relevant concepts and logical acumen, what is
metaphysically possible and impossible is not so available. Knowledge of the metaphysically necessary and possible is,
in general, a posteriori. Similarly, it is often suggested that essential properties show that we need to make a distinction
in kinds of necessity between metaphysical and conceptual necessity.
I think, as against this view, that it is a mistake to hold that the necessity possessed by Water = H O is different from
2
that possessed by Water = water , or, indeed, 2+2=4 . Just as Quine insists that numbers and tables exist in the very
same sense, and that the difference between numbers existing and tables existing is a difference between numbers and
tables, I think that we should insist that water's being H O and water's being water are necessary in the very same
2
sense. The difference lies, not in the kind of
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Necessary, modulo worlds where there is no water, that is to say. We will later switch to examples like If H2 O covers most of the Earth, then water covers most of the
Earth and All water is H2 O , where the proviso is not needed.
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See e.g. Peter Forrest, Universals and Universalisability , Australasian Journal of Philosophy, 70 (1992), 93 8, and, for an especially explicit discussion, Lockwood, Mind, Brain
and the Quantum, 21 3. However, Lockwood's remarks on p. 22 about the unmysterious nature of metaphysical necessity suggest some sympathy with the two-dimensional
approach I discuss below, an approach I see as opposed to the two senses view. See also A. C. Grayling, An Introduction to Philosophical Logic (Brighton: Harvester Press,
1982), ch. 3. For a recent paper in the philosophy of mind in which the two senses view plays a prominent role, see Stephen Yablo, Mental Causation , Philosophical Review,
101 (1992): 245 80, esp. 251 7.
70 CONCEPTUAL ANALYSIS AND METAPHYSICAL NECESSITY
necessity possessed, but rather where the labels a priori and a posteriori suggest it lies: in our epistemic access to the
necessity they share. As far as I know, Kripke does not address the two senses question directly, but it is worth noting
that he says that statements representing scientific discoveries about what this stuff is . . . are . . . necessary truths in the
strictest possible sense , and that they are necessary in the highest degree whatever that means , which suggest that
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he does not hold a two kinds of necessity view.
I have two reasons for holding that there is only one sense of necessity and possibility in play here. The first is
Occamist. We should not multiply senses of necessity beyond necessity. The phenomena of the necessary a posteriori,
and of essential properties, can be explained in terms of one unitary notion of a set of possible worlds. The
phenomena do not call for a multiplication of senses of possibility and necessity, and in particular for a distinction
among the possible worlds between the metaphysically possible ones and the conceptually possible ones.
The Occamist Reason
Take essential properties first. What convinces us that there are essential properties is the intuitive appeal of the claim
that, as we go from one possible world to another, there are certain changes that require us to say that we have a
different thing rather than the same thing with different properties in the two worlds. A difference in origin, for
instance, is said to require us to say that we have two different tables rather than the very same table but with a
changed origin. But, in explicating this, we do not appeal to a different sort of necessity. The possible worlds that figure
in the story that articulates how a property can be an essential property of x, namely, by being possessed by x in every
possible world in which x appears, are to be thought of in the same way whatever precisely that is as those that
figure in the story about the necessity of 2 +2 =4 . It is, for instance, supposed to be a priori accessible that a table's
identity cannot survive a change in origin as we go from one possible world to another.
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Kripke, Naming and Necessity, 125 and 99, respectively.
CONCEPTUAL ANALYSIS AND METAPHYSICAL NECESSITY 71
The phenomenon of the necessary a posteriori calls for more discussion. We need, it seems to me, to have before us
from the beginning two central facts. First, it is sentences, or if you like statements or stories or accounts in the sense
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of assertoric sentences in some possible language, that are necessary a posteriori. Secondly, the puzzle about the
necessary a posteriori is not how a sentence can be necessary and yet it takes empirical work to find this out. Russians
utter plenty of sentences which are necessarily true, and yet it takes many of us a lot of empirical work to discover the
fact. The puzzle is how a sentence can be necessarily true and understood by someone, and yet the fact of its necessity
be obscure to that person. And the reason this is a puzzle is because of the way we use sentences to tell people how
things are a matter we adverted to briefly in Chapter 2 when discussing the a priori.
Consider what happens when I utter the sentence, There is a land-mine two metres away. I tell you something about
how things are, and to do that is precisely to tell you which of the various possibilities concerning how things are is
actual. My success in conveying this urgent bit of information depends on two things: your understanding the
sentence, and your taking the sentence to be true. We have here a folk theory that ties together understanding, truth,
and information about possibilities; and the obvious way to articulate this folk theory is to identify, or at least
essentially connect, understanding a sentence with knowing the conditions under which it is true; that is, knowing the
possible worlds in which it is true and the possible worlds in which it is false; that is, knowing the proposition it
expresses on one use of the term proposition . This kind of theory in its philosophically sophisticated articulations is
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best known through the work of David Lewis and Robert Stalnaker. But it would, I think, be wrong to regard the folk
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