[ Pobierz całość w formacie PDF ]
expressed that determines that the sentence is necessary. And the important point for us is that this story about the
necessary a posteriori does not require acknowledging two sorts of necessity. The story was all in terms of the one set
of possible worlds.
An Unexpected Ally
I know from discussion that many insist that I have completely failed to learn the Twin Earth lesson. I reduce
important discoveries about necessity, namely, that it should be sharply divorced from a priority, and that conceptual
necessity is quite distinct from metaphysical necessity, to a linguistic phenomenon! So let me cite a piece of evidence
from the discoverer of Twin Earth. Hilary Putnam's recent reflections on Twin Earth include the following passage:
93
When terms are used rigidly, logical possibility becomes dependent upon empirical facts.
At first reading, this is a surprising remark. Surely, a fact about English usage which terms are rigid and which are
not is not relevant to the acceptability of the principle that something is possible if and only if it is necessary that it is
possible? The way to make sense of it, I submit, is as a claim about the sentences in which the rigidly used terms appear.
But it is hardly news, and anyway not something that needs support from Twin Earth parables, that the modal status of
a sentence depends on facts about word usage. It follows from the fact that words and sentences might have had
different meanings from those they in fact have. What is interesting, and what it did take Twin Earth considerations to
93
Putnam, Is Water Necessarily H2 O? , 62.
CONCEPTUAL ANALYSIS AND METAPHYSICAL NECESSITY 75
94
show, is that consistent with fixing what is required for understanding the sentence Water = H O , we can change its
2
modal status (that is, change the modal status of the proposition it expresses) by changing empirical facts. We can thus
make good sense of Putnam's claim by reading it in the style recommended by two-dimensional modal logic.
95
The Other Proposition Way of Saying What I Have Just Said
I said we can understand certain sentences, water sentences for example, by knowing how the proposition expressed
depends on context, and so do not need to know the sentences' truth-conditions. But to know how the proposition
expressed depends on context is to know truth-conditions in another sense of a sentence's truth-conditions. For
example, the knowledge required to understand Water covers most of the Earth can be given in the following array:
If H O is the watery stuff we are actually acquainted with, then Water covers most of the Earth expresses a
2
proposition that is true iff H O covers most of the Earth.
2
If XYZ is the watery stuff we are actually acquainted with, then Water covers most of the Earth expresses a
proposition that is true iff XYZ covers most of the Earth.
and, generalizing,
If . . . is the watery stuff we are actually acquainted with, then Water covers most of the Earth expresses a
proposition that is true iff . . . covers most of the Earth.
Although for each distinct, context-giving antecedent concerned with the relevant facts about how things actually are, a
distinct proposition is expressed by the sentence, simple inspection of the array shows that the sentence is true if and
only if most of the Earth is covered by the watery stuff of our acquaintance. So, in that sense, the understanding
producer of the sentence does know
94
Twin Earth considerations in their other possible world form, not their remote place in the actual world form.
95
I am much indebted to David Lewis, Pavel Tichy, and David Chalmers here.
76 CONCEPTUAL ANALYSIS AND METAPHYSICAL NECESSITY
when the sentence is true. Accordingly, we could say, following Tichy, Chalmers, Lewis, and Stalnaker among others,
that there are two propositions connected with a sentence like Water covers most of the Earth . The one we have been
calling the proposition expressed is the set of worlds at which the sentence is true given which world is in fact the
actual world; the other is the set of worlds satisfying the following condition: given that w is the actual world, then the
sentence is true at w. In this second case, we are considering, for each world w, the truth value of S in w under the
supposition that w is the actual world, our world. We can call this set of truth-conditions the A-proposition expressed
by S A for actual. In the case of the first proposition, however, we are considering, for each world w, the truth value
of S, given whatever world is in fact the actual world, and so we are considering, for all worlds except the actual world,
the truth-value of S in a counterfactual world. We can call this set of truth-conditions, the C-proposition expressed by
T C for counterfactual. Obviously, the A-proposition is an extension to sentences of the A-intension of terms, and
the C-proposition is an extension to sentences of the C-intension of terms, that we talked about in the previous
chapter. It is, I take it, the C-proposition that is normally meant by unadorned uses of the phrase proposition
96
expressed by a sentence when proposition is meant in its set-of-truth-conditions sense.
It is, as Stalnaker, Tichy, and Chalmers emphasize, the A-proposition expressed by a sentence that is often best for
capturing what someone believes when they use the sentence, and for capturing the information they seek to convey by
uttering a sentence. Thus, children who have not yet had the chemistry lesson in which they are told that water is H O,
2
but who understand the sentence Water covers most of the Earth , will use the sentence to express their opinion that
most of the Earth is covered by the watery stuff of our acquaintance. And, in general, it is the A-proposition we know
in virtue of understanding a sentence.
96
As opposed, for instance, to the sense in which propositions are thought of as individuated by the concepts that in some sense make them up. See e.g. Christopher Peacocke,
A Study of Concepts (Cambridge, Mass.: MIT Press, 1992). For a recent example of the use of proposition to mean what we are calling the C -intension (proposition) and
not the A -intension (proposition), see Adams and Stecker, Vacuous Singular Terms .
CONCEPTUAL ANALYSIS AND METAPHYSICAL NECESSITY 77
Thus, we have two superficially different but essentially identical accounts of the necessary a posteriori. One says a
[ Pobierz całość w formacie PDF ]