consequence of the
pressure of other bodies, immediately surrounding that which is felt
to be heavy.[13] For the only conceivable _actio in distans_ is that
which is mediated by thought, and it is only in so far as we suppose
matter to have in it a principle of activity like thought, that we can
accept such explanations of its motion. Again, while we must thus keep
our conception of matter clear of all elements that do not belong to
it, we must also be careful not to take away from it those that _do_
belong to it. It is a defect of distinctness in our ideas when we
conceive an attribute as existing apart from its substance, or a
substance without its attribute; for this is to treat elements that
are only separated by a "distinction of reason," as if they were
distinct things. The conception of the possibility of a vacuum or
empty space arises merely from our confusing the possible separation
of any mode or form of matter from matter in general with the
impossible separation of matter in general from its own essential
attribute. Accordingly, in his physical philosophy, Descartes attempts
to explain everything on mechanical principles, starting with the
hypothesis that a certain quantity of motion has been impressed on the
material universe by God at the first, a quantity which can never be
lost or diminished, and that space is an absolute plenum in which
motion propagates itself in circles.

Material universe a mechanism.

It is unnecessary to follow Descartes into the detail of the theory of
vortices. It is more to the purpose to notice the nature of the
reasons by which he is driven to regard such a mechanical explanation
of the universe as necessary. A real or substantive existence is, in
his view, a _res completa_, a thing that can be conceived as a whole
in itself without relations to any other thing. Now matter and mind
are, he thinks, such complete existences, so long as we conceive them,
as pure intelligence must conceive them, as abstract opposites of each
other; and do not permit ourselves to be confused by those mixed modes
of thought which are due to sense or imagination. Descartes does not
see that in this very abstract opposition there is a bond of union
between mind and matter, that they are correlative opposites, and
therefore in their separation _res incompletae_. In other words, they
are merely elements of reality substantiated