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claim. But he immediately (iv 206) goes on to claim more than this,
that our certainty transcends moral certainty; and it is clear that he
thinks that he can attach to the scientific system the degree of
certainty that can be attached, under God s guarantee, to the belief
that there is an external world at all. That is to say, the belief in the
scientific system is  or at least ideally can be  as certain as the
belief that there is anything at all for such a system to apply to.
science and experiment 253
This, however, could mean either one of two different things. It
could mean (i) that the degree of certainty that God s guarantee
offers is equal to that offered in the case of the belief in the external
world: on this line, there might be propositions other than the
belief in the external world which God had to guarantee to provide
certainty for science, but which he did indeed guarantee. Alter-
natively, it might mean (ii) that it is just the proposition about the
existence of the external world which is in question: once God has
guaranteed that, he has provided the guarantee for science.
In discussing these alternatives, I shall speak of God s  external
guarantee, meaning by that the guarantee that God supplies,
according to Descartes, for propositions which we are strongly dis-
posed to believe when we clearly and distinctly conceive them, but
with which clear and distinct perception by itself reveals consist-
ency, not truth. (We are not concerned here with any guarantee
extended by God to self-evident propositions or purely a priori
reasoning, the issue discussed in Chapter 7.) To illustrate the dif-
ference between the two views (i) and (ii), we can consider the total
claim made by the Cartesian physicist as consisting of the conjunc-
tion of four parts: (a) a set of principles, laws of nature; (b) deduc-
tions drawn from these by mathematical reasoning; (c) a claim that
(a) and (b) explain a set of phenomena, P; (d) a claim that P object-
ively exist in reality. The view (ii) allocates God s  external guar-
antee only to element (d); all of (a), (b) and (c), on this view, can be
known purely a priori. The alternative view (i) finds the  external
guarantee applicable elsewhere as well.
Let us start with the question of its application to (c). (c) is
ambiguous. It might indeed mean that certain purely logical rela-
tions obtained between the laws and P, as that certain mechanisms,
sufficient to generate P, are possible within the constraints of the
laws. In this sense we may allow that (c) can be known a priori. But
this does not yet serve to identify the explanation of P: we
encounter the familiar point that there is more than one way in
which P can come about, given the laws. When he has less than the
full set of phenomena to work on, these alternatives are real alter-
natives for the scientist, as we have seen. As he takes larger sets of
the phenomena, the alternatives, as we have also seen, narrow
254 science and experiment
down. They narrow down, however, only granted certain principles
of explanation, which use notions of simplicity, economy, etc. God
could, even within the limits of our a priori understanding, bring
about the phenomena in the world by a vast variety of arbitrary
mechanisms, which we would never be able to unravel. In order to
narrow down the explanations, we have to believe that he has not
done this, but has made the world as we, using principles of sim-
plicity, etc., and thinking as clearly as we can, will be disposed to
understand that it is made. This is not a purely a priori matter, and
requires another use of God s  external guarantee, as well as that
applied to (d)  the guarantee, that is to say, that what after con-
scientious investigation, using the best methods available to us, we
are compellingly but not demonstratively led to believe, must be
true.
Although it is not altogether easy to extract an answer from the
texts, I think that it is this last, more complex, account that best fits
Descartes s views. If we adopt it, we shall have applications of God s
 external guarantee to two beliefs: that the phenomena are really
there, and that the explanation we are led to, granted (a) and (b), of
those phenomena, in the light of canons of simplicity, etc., is the
true explanation. This will give us two areas in which the complete
physics will not be a priori in the strongest sense, identified above.
It leaves us with the claim that the element (a) of the physical
theory, the laws of nature themselves, can be known purely a pri-
ori. But is even this much a correct account of Descartes s thought?
Some of what he says strongly suggests that it is: so a passage of a
letter to More (5 February 1649: V 275, K 243, quoted below,
p. 259); Principles ii 64; and Principles iii 43. Yet not all these asser-
tions are quite so strong or unambiguous as they first look.
Principles ii 64 (quoted above, p. 243, in our first set of passages) is,
in the way already discussed, basically about the  object of physics,
and the mathematical conceptual apparatus of its demonstrations.
Principles iii 43, which has also already been quoted, p. 252, refers
to principia, principles, evidentissimè perspecta, literally  most
clearly seen through , tres-évidens in the French version; the article
as a whole, as we have seen, relates to the benevolence of God, and
it seems permissible to read this expression as referring to that
science and experiment 255
clear and distinct perception which reveals consistency rather than
truth, or at least as including that.
There is room for a suggestion (it cannot be stronger than that)
that Descartes did not regard his basic laws of nature, or all of them,
either as intrinsically self-evident, or as derivable by entirely
logical reasoning from self-evident metaphysical premisses. He
speaks of  deducing the laws of nature from considerations about
God s nature, in particular his immutability, but here we must
remember that weak sense of  deduce which we have already [ Pobierz całość w formacie PDF ]

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