c CUNYB/Clarke December, :
The ScientificEssaysand theDiscourse on Method obvious that the scientific essays are the main text, and their relative size
alone confirms that fact.More fundamentally, it is clear from the history
of its publication that theDiscoursewas planned merely as a Preface to the
scientific essays, and that it was written when the book was being printed
partly as a concession to Mersenne’s importunate requests for publication
of the underlying physical theory on which the essays were based.
None of these reasons, of course, will persuade most readers today to
change their reading habits and to read theEssaystogether with the famous
Preface on method. On the other hand, if one reads theDiscourseout of its
original context it is almost impossible to understand what the book was
about or why its author wrote it in the form in which it finally emerged
from the printer at Leiden in.For those who wish to understand
theDiscourse,itisnecessary to consider the essays for which it provided a
Preface.
The traditional distinction between catoptrics and dioptrics repre-
sented a division between studies of light reflection and light refraction
or, in simpler terms, between studies of mirrors and lenses. Descartes’
Dioptricsis designed as a discussion of the extent to which the invention
of telescopes or other lenses can assist human vision. In fact, however, it
is just as much a philosophical discussion of sensation and a reworking of
some of the themes about perception that had been presented in the initial
chapters ofThe World. Since he still had no definite plans for that book
in, Descartes offers readers a glimpse of his fundamental rejection
of scholastic theories of perception by including in theDioptrics,inthe
fourth discourse, a discussion of ‘The Senses in General’.
TheDioptricsrepeated an evasive ploy that had been first adopted almost
ten year earlier in theRules, when Descartes claimed that he did not need
to explain the true nature of light. Here again he says that he is concerned
only to explain how its rays ‘enter the eye and how they can be bent by the
different bodies that they encounter’ (vi.). This suggests that we think
of the action of light by analogy with how a blind man perceives objects
byusing a stick, or how wine tends to flow through holes in the bottom
of a barrel. The first analogy helps the reader not to think of light as a
something that moves from a visible object to our eye, just as nothing needs
to pass along a blind man’s stick in order for him to ‘perceive’ things. The
implication of the analogy is that there is no reason to assume that ‘there is
anything in these objects which is similar to the ideas or sensations that we
have of them’ (vi.). In this way, readers’ minds will ‘be delivered from