Descartes: A Biography

c CUNYB/Clarke     December, :

Retreat and Defence (–) 

This is an honest attempt to salvage the logic of confirmation from the

potentially lethal objection that it is fundamentally fallacious. The appeal

to a range of factors – the intelligibility of what he assumed, the small num-

ber of hypotheses used, and the fact that the same assumptions explain

a wide range of apparently disparate phenomena (rather than having a

new hypothesis for each thing that needs to be explained) – was the

only recourse available in the seventeenth century in defence of scien-

tific method.

Mersenne raised the same kind of general problem about confirmation,

and he was rewarded with a summary of all the strategies already offered

to Morin. Descartes’ reply is worth quoting at length:

Yo uask if I claim that what I wrote about refraction is a demonstration. I think it

is, at least insofar as it is possible to provide a demonstration in this subject without

having first demonstrated the principles of physics by metaphysics (something that I

hope to do some day...), and insofar as any other question in mechanics, or optics, or

astronomy, or another subject which is not purely geometrical or arithmetical, has ever

been demonstrated. However, to ask me to provide geometrical demonstrations in a

subject that depends on physics, is to ask me to do the impossible....It is sufficient, in

subjects such as these, if the authors presuppose certain things that are not manifestly

contrary to experience, and if they reason coherently and without making logical

mistakes, even if their assumptions were not strictly true. Now what I claim to have

demonstrated concerning refraction does not depend on the truth about the nature

of light...butonly on the assumption that it is an action or a power that obeys

the same laws as local motion....As for those who are content to say that they do

not believe what I wrote, because I do not deduce it from certain assumptions that

I did not prove, they do not know what they are asking for, nor what they ought to ask

for. (ii.–)

Descartes clearly distinguishes here between the kind of demonstration

or proof that is appropriate to mathematics and the kind that it is possible

to realize in physics.Once that point is conceded, however, there is little

effort made to explain the degree of certainty that can be expected in optics

or meteorology. Evidently, Descartes wanted to claim as much certainty

as possible for the results of his work, and to challenge critics to explain

what alternative methods they might use.

Morin and Mersenne both raised two types of objection, one of which

was the general question just mentioned about the logic of confirmation.

The other type of objection involved detailed reservations about spe-

cific explanations. Morin accepted that it would have been impossible for

Descartes to give an adequate explanation of any optical phenomenon as