304 Kant: A Biography
"must be regarded as force." Therefore he also believed that he had "ban¬
ished from natural science as an empty concept" the idea of solid or ab¬
solute impenetrability.^125 Because the "mechanical mode of explanation ...
under the name of atomistic or corpuscular philosophy" depends on the
concept of absolute impenetrability, that is, absolute hardness, he also be¬
lieved that he had disproved it. This theory might be "very convenient for
mathematics," and it might have been very influential from Democritus to
Descartes, but it does not make sense.^126 This of course also has conse¬
quences for Kant's understanding of the theory of Newton and his fol¬
lowers. Since this theory involves atomism, Kant must reject this aspect
of Newtonianism. But Kant's idea of force captures another aspect of New¬
ton's thought, namely, his insistence on universal gravitation. Kant may be
seen as trying to save the Newtonians from their own misunderstandings.^127
In the third chapter, on the metaphysical foundations of mechanics, Kant
defines matter as movable just insofar as it has "a moving force."^128 That
is, he supplements the definition of the second chapter, which would hold
even if matter were at rest. It is here that Kant introduces and attempts to
prove the laws of motion. Though Kant's laws of motion are related to New¬
ton's three laws of motion, Propositions 2, 3, and 4 do not correspond ex¬
actly to Newton's First, Second, and Third Laws of Motion. Although Kant
calls his Proposition 2 the "first law of mechanics," it is not Newton's First
Law — nor his second or third — but the principle of the conservation of
"quantity of matter (i.e., mass). Kant's Proposition 3 (what he calls the
"second law of mechanics") is almost the same as Newton's First Law (the
principle of rectilinear inertia). One of the differences between Newton and
Kant is that Newton talks about force, whereas Kant says "cause." Kant's
Proposition 4 is almost the same as Newton's Third Law. There is no ex¬
plicit Kantian demonstration of Newton's Second Law.
Kant formulates his laws within a metaphysical rather than a scientific
context. This metaphysical context relies heavily on his own epistemolog-
ical views as developed in the first Critique.^129 In Chapter 4, "The Meta¬
physical Foundations of Phenomenology," Kant defines matter as the
movable "insofar as such can be an object of experience."^130 Relying on the
distinction between empirical and absolute space, he argues that absolute
space is "nothing at all." All motion must have reference to some other
empirically given matter. Therefore, absolute motion is absolutely impos¬
sible. On the other hand, if all motion must have reference to some other
empirically given matter, then this also means that we cannot have a "con¬
cept of motion or rest in relative space and valid for every appearance."^131
Yet we need such a concept to make sense of the notion of relative space.