Descartes: A Biography

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 Descartes: A Biography

all those little images flying through the air, called “intentional species ”,

which worry the imagination of philosophers so much’ (vi.). Instead,

Descartes asks his readers to think of light simply as a feature of subtle

matter best described as a tendency to move, and to assume that this

tendency is transmitted in straight lines.

This almost instrumentalist attitude allows Descartes to focus on the

geometry of how light rays are bent. He presents a mathematical analysis of

how light is reflected from smooth surfaces, such as mirrors, and how it is

refracted when it travels from one medium to another (e.g., when it passes

from the air into glass). In the case of reflection, Descartes argues that the

angle of incidence is equal to the angle of reflection. In the case of refrac-

tion, he develops a mathematical analysis that concludes with the sine law

of refraction, the same conclusion that resulted from independent work by

the Dutch physicist Snellius. Having established these laws of reflection

and refraction, Descartes needs to describe the anatomy of the eye.

It is hard to avoid the conclusion that the description of a cross-section

of the eye, presented in the third discourse of theDioptrics,isbased at

least in part on anatomical dissections that Descartes had done during

the previous years. Without using technical terms such as ‘iris’, ‘pupil’,

or even ‘optical lens’, he describes the various parts of the eye, including

what he calls the ‘optical nerve’, with only enough detail to make it possible

forreaders to follow the discussion in subsequent chapters. However, as

promised to Huygens, he does provide many diagrams to illustrate the

points being made in the text. This is in stark contrast, for example, to

a book on the same subject written by one of his contemporary critics,

Vopiscus Fortunatus Plemp (–). Plemp had studied medicine,

first at Leiden and later at Bologna, one of the best medical schools in

Europe. Having practised medicine at Amsterdam for six years, he was

appointed professor of medicine in Louvain. In, Plemp published

hisOphthalmographia, or the structure, action and use of the eye according

to the common opinion of physicians and philosophers.This relatively short

treatise explains the functioning and diseases of the eye without using even

onediagram to illustrate the text.

The fourth, fifth, and sixth discourses of theDioptricsprovide a sum-

mary of the Cartesian theory of vision and, by extension, of sensation in

general. Descartes concedes at the outset that ‘it is the mind which senses

and not the body’ (vi.), or, more exactly, it is the mind insofar as it is

‘in the brain, where it exercises the faculty called common sense’ (vi.).