Benedict de Spinoza, THE ETHICS
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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
substance: that which constitutes the actuality (formam) of an
individual consists (by the last Def.) in a union of bodies; but
this union, although there is a continual change of bodies, will
(by our hypothesis) be maintained; the individual, therefore,
will retain its nature as before, both in respect of substance
and in respect of mode. Q.E.D.

Lemma V. If the parts composing an individual become greater or
less, but in such proportion, that they all preserve the same
mutual relations of motion and rest, the individual will still
preserve its original nature, and its actuality will not be
changed.

>>>>>Proof--The same as for the last Lemma.

Lemma VI. If certain bodies composing an individual be compelled
to change the motion, which they have in one direction, for
motion in another direction, but in such a manner, that they be
able to continue their motions and their mutual communication in
the same relations as before, the individual will retain its own
nature without any change of its actuality.

>>>>>Proof--This proposition is self-evident, for the individual
is supposed to retain all that, which, in its definition, we
spoke of as its actual being.

Lemma VII. Furthermore, the individual thus composed preserves
its nature, whether it be, as a whole, in motion or at rest,
whether it be moved in this or that direction; so long as each
part retains its motion, and preserves its communication with
other parts as before.

>>>>>Proof--This proposition is evident from the definition of an