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we say about the claim that enough information about individual heights a
priori entails what the average height is? The wrong thing to say is that we
have discovered a major issue that requires adjudication. Rather, we have a
terminological issue that requires us to draw a distinction: if information about
individual heights excludes the stop information, you cannot a priori deduce
average heights from such information; if it includes it, you can.
A similar situation applies in the case of the a priori passage principle. Rich
enough physical information might be read so as to include a stop clause, or
it might be read so as to exclude a stop clause. Read without the stop clause,
the principle is certainly false. I will make the point first for a toy example.
Consider a world w3 that contains three electrons whose nature is as conceived
in current physics, and nothing further. Whatever may be true of our world,
we can all agree that physicalism is true at w3, and we can all agree that w3
does not contain any shopping centres. Can we a priori deduce that fact from
a rich enough account of the physical nature of w3? The obvious answer is yes
someone who thinks that three electrons might make up a shopping centre
does not have our concept of a shopping centre. However the answer is no,
unless the stop clause, carried in this case by the words and nothing further ,
is included as part of the rich enough physical story about w3. A world with
three electrons might have much else besides, including shopping centres.
We defenders of a priori passage, or at least all the ones I know, are
explicit that the physical information which we claim a priori entails where
the shopping centres are, who is thinking what, when inflation peaked, etc.,
must in general include the stop clause. The situation is as follows. Exclude
the stop clause in what is meant by a rich enough physical account and you
get a principle that is certainly false and which no-one defends; include the
stop clause and you get a principle that has some chance of being true and is
the one, give or take points of precise formulation, that some people, includ-
ing me, Chalmers (1996) and David Lewis (1994), accept. This point will be
important later.
From H2O to water 87
3 The target argument
Consider [under the assumption that we have the percentage correct, it will
be important to suppose that (a) below is true]:
(a) Sixty per cent of the earth is covered by H2O.
Therefore,
(d) Sixty per cent of the earth is covered by water.
The passage from (a) to (d) is not a priori, although it is necessarily truth
preserving. However, many have supposed that something like (formulations
vary)
(b) Water is the stuff that plays the water role.
is a priori, where the water role is spelt out in terms of being potable, odourless,
falling from the sky, being the stuff that makes up various bodies of liquid of
our acquaintance or in some ostended set of samples, etc. In short, the water
role is spelt out in terms of the reference fixers for water , and the case for
(b) s a priori status rests on the general thesis that N = the F is a priori when
F specifies the reference fixers for N .6 If (b) is a priori, then the conjunction
of (a) with the empirical truth that
(c) H2O is the stuff that plays the water role.
means that we have two H2O truths that together a priori entail (d).7
What is the significance of this result? It tells us that it is a mistake to infer
from the fact that Any water is H2O is necessary a posteriori that there is no
a priori passage from the way things are framed in terms of H2O to the way
they are framed in terms of water. Of course, this presupposes that we have to
hand a way of spelling out the water role in (b) which plausibly both makes
(b), or something suitably like it, a priori and does not contain the term water
or an equivalent. I will follow the practice of using the term water role , or
waterish (and heatish and heat role when discussing the case of heat in
gases), but it is important that such expressions be viewed as shorthand for
longer expressions that do not contain water (or heat ).8
I will address the following criticisms that Block and Stalnaker make of
the argument.
(1) They argue that the definite description in (b) makes trouble for the
claim that (b) is a priori and that this matters.
(2) They argue that the definite description in (c) means that the argument
88 Frank Jackson
is, as a matter of principle, unsuitable to be a suggestive model for the
discussion of the a priori passage principle in general.
(3) They argue that the example of scientific reductions gives no reason for
holding that (b) is a priori.
I will address these criticisms more or less in that order. I will conclude
with a short statement of the positive reason for holding that something like
(b) has to be a priori. When you hear the reason, you will understand why I
keep on saying that something like (b) is a priori. I sometimes (understandably)
meet the complaint that I should be able to say exactly what is a priori I can
hardly plead lack of empirical data to excuse my vagueness but we will see
why it has to be something like (b) that is a priori.
4 Is it a priori that water is the stuff that plays the
water role?
Block and Stalnaker point out, correctly, that the uniqueness of the stuff that
plays the water role is important to the target argument. For example, from
the premises that H2O is one of the kinds that plays the water role, and that
water is one of the kinds that plays the water role, nothing follows a priori
about the distribution of water from the distribution of H2O. Consider the
following analogy: from the fact that drug X is a cure for malaria and drug Y
is a cure for malaria, it does not follow that X = Y.
Block and Stalnaker see the need for uniqueness as making serious trouble
for the target argument in two different ways. One way arises from doubts
about its being a priori that water is the waterish stuff, that is the unique
waterish stuff. Block and Stalnaker point out that there is a case to be made
that there might have been, in the sense that it is conceptually possible,
more than one kind of water. Water might have turned out to be like jade,
something that comes in two kinds.9 In fact, they go further and suggest that
it is conceptually possible that water is the role property. On the face of it,
this cannot be right: it is the occupants of the water role that do the things
that we all agree water does, but I take it they mean that it is conceptually
possible that there be indefinitely many different kinds that play the water
role consistently with each being water.
I think they are right that it is conceptually possible that water is like jade,
although I think they go too far when they suggest that it is conceptually pos-
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