motion can only have led to
continued motion; the state of rest therefore must have resulted
from something, which was not in A, namely, from an external
cause determining A to a state of rest.

Axiom I.--All modes, wherein one body is affected by another
body, follow simultaneously from the nature of the body affected
and the body affecting; so that one and the same body may be
moved in different modes, according to the difference in the
nature of the bodies moving it; on the other hand, different
bodies may be moved in different modes by one and the same body.

Axiom II.--When a body in motion impinges on another body at
rest, which it is unable to move, it recoils, in order to
continue its motion, and the angle made by the line of motion in
the recoil and the plane of the body at rest, whereon the moving
body has impinged, will be equal to the angle formed by the line
of motion of incidence and the same plane.

So far we have been speaking only of the most simple bodies,
which are only distinguished one from the other by motion and
rest, quickness and slowness. We now pass on to compound bodies.

Definition.--When any given bodies of the same or different
magnitude are compelled by other bodies to remain in contact, or
if they be moved at the same or different rates of speed, so that
their mutual movements should preserve among themselves a certain
fixed relation, we say that such bodies are in union, and that
together they compose one body or individual, which is
distinguished from other bodies by the fact of this union.

Axiom III.--In proportion as the parts of an individual, or a
compound body, are in contact over a greater or less superficies,
they will with greater or less difficulty admit of being moved
from their position; consequently the individual will, with
greater or less difficulty, be brought to assume another form.
Those bodies, whose parts are in contact over large superficies,
are called hard; those, whose parts are in contact over small
superficies, are called soft; those, whose parts are in motion
among one another, are called fluid.

LEMMA IV. If from a body or individual, compounded of
several bodies, certain bodies be separated, and if, at the same
time, an equal number of other bodies of the same nature take
their place, the individual will preserve its nature as before,
without any change in its actuality (forma).

Proof.--Bodies (Lemma i.) are not distinguished in respect of
sub