Student and Private Teacher 91possess, he tries to show that they may therefore contain an internal prin¬
ciple that causes them to exert force. Such a body may "increase within it¬
self the force which has been awakened in it by the cause of an external
motion."^137 Kant calls the motion caused by such an internal principle a
"free motion," that is, a motion whose speed always remains the same. The
measure of the speed of bodies in free or infinite motion, as he also calls
it, is living force. While the measure of all other motions is momentum,
free motion must be understood along Leibnizian lines. What is impor¬
tant to Kant is that living force is possible only if there are free motions.^138
Yet we cannot prove that there are free motions. We can only assume them
as a hypothesis. The theory of living forces is also only a hypothesis, and
this is, as Kant points out, all that Leibniz meant to say in the Tkeodicee.^139
Kant's new theory turns out to be a defense and modification of Leibniz's
theory of living forces.
It also seems to be related to Newton's ideas about "active force." Vis
inertia was not sufficient for Newton to explain the variety (or perhaps
better, the quantity) of motion, which is constantly decreasing and always
"upon the decay." We must therefore, he argued, postulate active principles,
which explain why the world does not come to a standstill. Newton could
never decide "what that principle is, and by means of [what] laws it acts on
matter." It was "a mystery," and he did not know how it was related to mat¬
ter.^140 Kant thought that he could connect this thought with Leibnizian
ideas about living forces.
The doctrine of living forces was connected to the theory of monads.
Leibniz believed that a completely materialistic or mechanistic explana¬
tion of the phenomena was impossible and therefore posited form, en-
telechy, and force as an internal principle of substances. Kant accepted this
view. When he differentiates between mathematical bodies and natural
bodies, and when he assigns an internal force to natural bodies that enables
them to have free motion, he seems simply to be following Leibniz, but he
is not. Rather, he is following, or perhaps better, developing, Baumgarten,
a Wolffian who moved closer to Leibniz than did any of his other Wolffian
contemporaries. Baumgarten tried to defend preestablished harmony
against physical influx by giving up the claim that monads do not act on
each other. Like Kant, he claimed "monades in se mutuo influunf ("mon¬
ads influence each other").^141 This is - or it seems to be - different from
what Leibniz proposed. Leibniz did not believe that monads interact, or
that they stand in real external relations with each other.
Though some of Kant's (and Baumgarten's) observations were meant