Descartes: A Biography

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 Descartes: A Biography

trees, while the former is composed of parts that are more lubricated, similar to those

that have the shape of eels. However, experience shows that water is more fluid and

that it freezes less easily than oil. Therefore,... (i.–)

Descartes leaves the final sentence dangling with an incomplete ‘there-

fore’, and then gives three similar incomplete syllogisms. He concludes

that, although each syllogism when considered separately provides only a

probable conviction, ‘all of them taken together provide a demonstration’

(i.). The logical difficulties in dispute between the two philosophers

are camouflaged by the phrase ‘that is a sign’, because it allows Descartes

to reverse the two parts of the first sentence (which he hopes his scholastic

critic will accept as a major premise), and to recast his argument in a valid

form.However, once the sentence is rewritten without ambiguity, it is clear

that he is arguing as follows: ‘Ifwater were composed of fluid particles like

eels, etc.,thenwater would freeze more easily than oil. Water does freeze

more easily than oil. Therefore,...’AsDescartes knew very well, noth-

ing follows fromthatsyllogism about the truth of the first assumption. It

suffers from the same logical defect that emerged earlier with the amateur

sleuth.

This problem was made explicit by Jean-Baptiste Morin, a professor

of mathematics at the Coll`ege de France. Morin wrote a very friendly but

detailed critique of the Cartesian theory of light, in February.He

quoted extensively from various parts of Descartes’ text which had sug-

gested that light is a movement, a mere tendency to move, or an ‘action’

that tends to cause motion, and that this tendency affects a subtle matter

that is said to fill up all the apparently empty places between larger particles

of matter, including the spaces between particles of translucent bodies.

Morin also aligned himself with Descartes as an anti-establishment sci-

entist, by reminding him of his own [Morin’s] earlier failure to persuade

a royal commission that he had solved the problem of how to calculate

longitudes at sea by relying on lunar observations. Having praised the

excellence of Descartes’ work in mathematics, he contrasted it with his

work in physics, where he should not be surprised to have critics because

he had withheld knowledge of ‘the principles and universal notions of

your new physics, the publication of which is passionately desired by all

the learned’ (i.).With these preliminary expressions of admiration

and caution in place, Morin identified the central issue about the logic of

confirmation: