Descartes: A Biography

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Retreat and Defence (–) 

that he had outlined his theory of light in more detail there than in the

Dioptrics.However, it is equally clear thatThe Worlditself had also been

drafted as a hypothetical enterprise, and that the style of argument used to

support its theory of light was exactly the same as that being questioned by

critics of theEssays. The appeal to something inThe Worldthat his readers

could not check – not even his most trusted friends, such as Huygens –

merely camouflaged the fact that Descartes could not provide the kind of

confirmation of his theories that he describes in the letter to Vatier as ‘a

priori’ (i.).

This raises a question about what Descartes thought he had achieved by

linking all his thoughts together into a coherent network of beliefs. There

was no suggestion that any of his scientific opinions about specific items –

such as the circulation of the blood, the explanation of why the heart beats

regularly, the nature of light, and the explanation of colour perception –

could be deduced in some rigorous way from axioms. Nor did he claim that

beliefs about the material world are intrinsically uncertain and require a

metaphysical foundation. Descartes did mention, when writing to Vatier

about proving God’s existence, ‘the uncertainty of all the knowledge we

have of material things’ (i.). However, this was not an endorsement of

a general scepticism about empirical beliefs. Descartes supported claims

about all the scientific items that he discussed during this period by ref-

erence to experiments and observations. For example, in one of his letters

to Plemp (February), he not only describes an earlier vivisection

onarabbit’s heart but also says that he repeated the experiment as he was

writing the letter (i.).The problem that emerged here was one that

had been recognized for centuries, and Descartes was trying to evade it by

appeals to the coherence of his belief system. It arose from the indirectness

with which theories are confirmed by the evidence that is used to support

them.

A Theory of Confirmation

There is a form of argument that is so obviously invalid that it had its own

proper name for centuries before Descartes.The mistake can best be seen

from an example. An amateur sleuth might argue as follows, on finding

the body of an apparent murder victim called Murphy. ‘If Murphy had

been killed by Estragon, then Murphy would be dead. Murphy is certainly

dead, since I just found his dead body. Therefore, Estragon killed him.’