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ERIC PALMER

How coherent, and how useful, is Descartes's conception of vacuum in the
Principles? I will begin by backtracking: in the World, written most of a decade ear-
lier, Descartes has much to say about macroscopic physical vacuum. He models the
created universe as a finite container in which matter is situated, and his arguments
are meant to indicate that the container is full to its boundaries in a continuously
connected medium, so that no vacuum is present, and light can travel as pressure in
the medium (AT XI, 32, 86). ¹ Though he suggests his argument is not conclusive,
Descartes's well-known treatment of the wine cask is a noteworthy prop: wine will
not flow from an unstopped hole in a full cask unless a vent is provided to allow air
to take its place since the universe is "as full as can be, and the part of the air whose
place the wine would occupy if it were to flow out can find no other place to occupy
in all the rest of the universe" (AT XI, 20).

As he does in the World, Descartes maintains in the second part of the Principles
that "in every case of motion, there is a complete circle of bodies moving together,"
and he suggests that bodies that our senses cannot detect fill in the spaces as others
separate. Descartes explains as much in his discussions of the empty jug (which is
not empty but full of air) and of rarefaction, which is taken to be the process of a body
enlarging by incorporating other matter, much as a dry sponge grows as it incorpo-
rates liquid (2:33, 17, 6). He argues that vacuum is unnecessary for motion in a non-
atomistic universe and, as before, is a barrier to the passage of light (2:33, 3:53ff).

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