Descartes: A Biography

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The Quarrel and Final Rift with Regius 

The enforced revisiting of Roberval’sAristarchusprovided Descartes

with another opportunity to articulate more clearly than on many other

similar occasions his understanding of what counts as an explanation of a

physical phenomenon and, by implication, what does not count as such.

He wrote to Mersenne, in April:

Whenever we assume something in order to explain something else, the assumed

reality must always be more probable, more evident, and simpler, or it should be

better known in some way. Otherwise, it could not enlighten us about the thing to be

explained. However, if one were to assume, for each of the things that one wishes to

explain, not only the same number of equally unknown things but many more less

credible things, and if moreover what one wished to explain by such assumptions did

not follow from them, one should surely not claim to have achieved anything worth

while. (iv.)

Descartes illustrates this kind of mistake by describing as ‘very absurd’ the

assumption that ‘there is some property in every part of the earth’s matter

in virtue of which they are drawn to each other and attract each other’,

and that there is another property in every part of the same matter with

respect to other particles that does not impede the first property. This

provoked the usual Cartesian objection: that it was equivalent to thinking

of each part of matter as having a soul, and even a number of souls, by

which each part could know what was happening in distant places and

exercise its powers there.

Roberval’s interventions, either about the Pappus problem or about

the calculation of pendulum oscillations, were dismissed by Descartes as

worthless. It seems clear, despite their friendly meeting in Paris two years

before, that Descartes’ opinion of the professor of mathematics at the

College de France had not changed since` .ToMersenne he wrote:

‘I shall tell you, between ourselves, that I have so many proofs of the

mediocrity of the knowledge and intelligence of its author [Roberval], that

Iamamazed at the reputation he has acquired in Paris.’As one might

expect, Roberval dismissed Descartes’ explicit appeal to observational

evidence and claimed that ‘reason and experience support my side. If Mr.

Descartes does not wish to accede to either one, I leave it to others to

decide what to expect from him.’This dispute was predictably doomed

to irresolution. Descartes suggested to Cavendish that he ask Roberval

forthe correct analysis of the problem; he himself could not contribute

further, and eventually Cavendish could take his pick between Descartes’

analysis and that of Roberval.In fact, Descartes was so reluctant to think