Descartes: A Biography

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AFabulous World (–) 

manuscript survived long enough to be partially published, in French,

asThe Treatise on Lightin.Inthe same year,The Treatise on Man

was published (also in French).Three years later, a more complete edi-

tion that included both treatises was published asLe Monde(The World).

Descartes describes in retrospect, in theDiscourse,howhe had decided to

limit the contents ofThe Worldto a few sample problems:

I was afraid that I could not include in my discourse everything that I had thought

about. Thus I undertook to explain in it reasonably fully only what I understood about

light and, at the same time, to add something about the sun and the fixed stars, because

most light results from them; about the skies, because they transmit the light; about

the planets, comets and the earth, because they reflect it; and in particular about all the

bodies that are on earth, because they are either coloured, transparent or luminous;

and finally, about human beings, because they are the perceivers of light. (vi.)

The Worldraised one of the central issues in seventeenth-century theory

of knowledge, in its very first chapter, concerning the possible difference

between the way in which things are in reality and the way in which we

perceive them. This question had already been raised by Galileo in.

Its significance for the new sciences is difficult to exaggerate.

The spontaneous or na ̈ıve interpretation of our own perceptions is that

the realities we perceive are exactly as they appear to us. This was the view

that had been adopted since classical times, and which was expressed, for

example, by Quintilian, an author who was prominently on the curriculum

at LaFl`eche: ‘We may regard as certainties, first those things which we

perceive by the senses, things for instance that we hear or see.’If this is

questioned, we can no longer assume any direct access to reality, even in

the simple perception of familiar things. Descartes addresses this question

in the first sentence ofThe World. ‘Since my plan here is to discuss light,

the first thing that I want to bring to your attention is that there may be a

difference between our sensation of light, i.e. the idea which is formed in

our imagination by means of our eyes, and whatever it is in the objects that

produces that sensation in us, i.e. what is called “light” in the flame or the

sun’ (xi.). Descartes illustrates his challenge by a number of persuasive

examples, one of which had earlier been used by Galileo when he discussed

the same question. If a child has a tickling sensation as a result, for example,

of someone rubbing a feather against their lips, it would be an obvious

mistake to assume that there is some quality in the moving feather that is