Descartes: A Biography

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The Principles of Philosophy() 

vortex, in the centre of which is the Sun’ (III,). This released him to

speculate about the nature of light, the stars, magnetism, and all the nat-

ural phenomena that had occupied his attention for almost fifteen years.

The final paragraphs of Part IV address the niggling worry, raised even by

sympathetic readers of theEssays, that his hypotheses might turn out to

be false. Descartes bets both ways on his physical theory, and still leaves

open the possibility that it may be false if it contradicts the teachings of

the church.

He argues in paragraphthat his hypotheses are ‘at least morally

certain’, that is, that ‘they have the kind of certainty required for daily

living’ (IV,). The concept of ‘moral certainty’ was already familiar in

the seventeenth century. If wine is bought from a reputable producer, it is

not necessary to get a chemical analysis before drinking a glass of it. Most of

what we do in our daily lives relies on a degree of certainty that is far short

of proof, but is sufficient to make it possible to carry on without undue

anxiety. Descartes gives another example to support his understanding of

the degree of certainty that could be claimed for his hypotheses. If one

imagines a letter written in code, and if one discovered a translation manual

that turns unintelligible strings of signs into meaningful words, one could

feel confident that the translation was correct, even if it were discovered

merely by conjecture. Likewise, ‘those who notice how many things about

the magnet, fire, and the whole fabric of the universe are deduced here from

so few principles...will acknowledge that it could hardly have happened

that so many phenomena were consistent with those principles if they were

false’ (IV,).

Stepping up the pressure, Descartes argues in the next paragraph

that some of his principles are ‘absolutely, and more than just morally,

certain’. He is not very specific about which claims ‘even about natu-

ralphenomena’ enjoy this kind of metaphysical certainty, and he con-

cludes that ‘at least the more general things that I wrote about the world

and the Earth can hardly be explained otherwise than as I have done’

(IV,). The final paragraph of the book, perhaps the most telling

symptom of his continuing concerns about censorship from Rome, is as

follows:

Nonetheless, mindful of my weakness, I affirm nothing. However, I submit all these

things both to the authority of the Catholic Church and to the judgment of more

prudent men. I do not wish anything to be believed by anyone, unless they are convinced

by evident and invincible reason. (IV,)