98 Chapter 6
string is the simplest example, reduced to a single dimension, of vibrating membranes and
solids. Thus, this problem emergent from musical experience was foundational for the
generalization of mechanics from point bodies to continuous media.
In this nexus of questions, the issue of the vacuum takes an important place; we have
seen, in the moment-to-moment byplay of Descartes ’ s thoughts, as he set them down to
Mersenne, his alternate consideration of and doubt about the nature and possibility of
motion in a vacuum. At first, he seems to resist Aristotle ’ s famous arguments against the
possibility of a void. Yet Descartes seems increasingly troubled by the status of a void vis
à vis the resistance of air or other media. The unfolding dialectic of this letter offers evi-
dence that Descartes ’ s rejection of the vacuum emerged in the context of this musical-
physical problem , one at least of the issues that moved him.
We can best see where this process took him by examining the text of Le Monde de Mr.
Descartes, ou le Trait é de la lumi è re et des autres principaux objects des Sens ( The World
of Mr. Descartes, or the Treatise on Light and on the Other Principal Objects of the
Senses ), as he assembled it during 1629 – 1633. His subtitle signals that, though light is his
subject, his design encompasses a sketch of the whole visible universe and all the “ objects
of the senses. ” By labeling his treatise merely an imaginary vision, he seeks a safe way
to present a new cosmology that could escape controversy or censure. Throughout this
work, sound is the hidden thread that helps him find and state his new worldview. At the
very beginning, Descartes distinguishes between “ our sensation of light ” and “ what is in
the objects that produces that sensation. ” For him, sound is essentially like touch, the sense
“ thought least misleading and most certain, ” yet “ even touch causes us to conceive many
ideas that in no way resemble the objects that produce them. ” Choosing an example that
makes one wonder about his military experiences, Descartes notes that in the heat of battle
a soldier might think he had been wounded, though “ what he felt was nothing but a buckler
or a strap. ”^34 Perhaps this was as close as he came to being wounded in action.
Descartes uses sound, understood as “ a certain vibration of air striking against our ears, ”
as a template to form his understanding of light and the cosmos, which he conceives as
an infinitely divisible fluid continuum. His reluctance to accept a void, which emerged in
his letters, eventuates in his view (expressed in Le Monde ) that the continuous world-fluid
is not finally atomic and hence does not admit of any void spaces, however small. As
critical as he was, Descartes finally found himself on the same side as Aristotle not only
because of the metaphysical problems of ascribing any positive properties to pockets of
emptiness but because he considered that sound must travel in a continuous medium (as
in his image of the spreading circles in a pond) that will not tolerate interruption by voids.^35
Thus, in his figures Descartes represents the minute “ parts ” of his fluid cosmos by little
balls, which model the interaction of the fluid on the small scale, not really atoms in any
physical sense. In figure 6.4 , the fluid can move by the transmission of motion from one
layer of balls to the next, each relaying the “ touch ” to the next by a process that is essen-
tially the same as his understanding of sound. He uses exactly the same pictures to explain