Descartes: A Biography

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Magic, Mathematics, and Mechanics 

anumber of steps that will conclude with a question about the nature of

light. Once this point is reached, they are forced to realize that light is a

natural power, and, if they cannot make progress in understanding light

byexamining it directly, they may understand it indirectly ‘by analogy’

with other familiar natural powers (x.), all of which are known by

experience.

This analogical approach matches the suggestions outlined in Rule

forunderstanding how we perceive things as being coloured, and it borrows

onDescartes’ earliest reflections in his notebooks on the way in which we

conceive of immaterial things by analogy with familiar material realities.

In Rule,herecommends that we avoid addressing directly the question

about how colour perception occurs, for we do not know enough to provide

an adequate explanation. Instead, we should model colours onto shapes,

because we can understand shapes very well, and there are as many shapes

available as there are colours in need of explanation (x.). This is the

same point that was made, with a more general application, inOlympics:

Just as the imagination uses shapes to conceive of bodies, in the same way the intel-

ligence uses certain sensible bodies to shape spiritual things, such as the wind and

light....Man knows natural things only by analogy with those that fall under his

senses. We even think that those who philosophize best, with greater truth, are those

who most successfully assimilate the things that are sought with what is known to the

senses. (x.,–)

These concessions to the necessity of experience, both in conceptualizing

the realities that we investigate and in gathering basic information about

them, remained inadequately integrated into a theory of knowledge that

still resonated, in, with a model of scientific knowledge that had

originally been borrowed from Euclid’s geometry. Descartes, in theRules,

thus repeats what Aristotle had claimed in thePosterior Analytics, that

‘every science is certain and evident knowledge’ (x.), and that the only

reliable way of acquiring such knowledge is by ‘intuition and deduction’

(x.). The rhetoric of certainty, of building knowledge on firm founda-

tions through a series of deductive steps, leaves unchallenged a model of

scientific knowledge that was about to be superseded by the overwhelming

experience of empirical scientists.

One reason why Descartes may have abandoned theRules, therefore, is

that they were too general to provide any specific advice about, for example,

how to develop theories in optics or physiology, and that they required a