1 The Birth of Science - MSRI

Plutarch, in the De facie, writes: 53
10.3 The Principle of Inertia 245
And further, to help the moon, that it may not fall, there is its motion
itself and the whizzing nature of its rotation, just as objects placed in
a sling are prevented from falling by the circular motion. For each
body is guided by motion according to nature, if it is not turned
aside by something else. For this reason the moon does not follow
its weight, which is cancelled by the counterweight of the rotation.
There would perhaps be much greater reason to marvel if it kept motionless
and still like the earth. 54
One should not pay attention to philosophers if they want to ward
off weirdnesses with weirdnesses and, to fight the astonishments
of one doctrine, make up things even more weird and astonishing.
Take the folks who introduced the thrust toward the center. What
weirdness is missing there?. . . Not that multi-ton, white-hot boulders
thrust through the depths of the earth, upon reaching the center,
should stay still with nothing touching or supporting them; nor
that if thrust down with impetus they should overshoot the center
and turn back again and keep bobbing back and forth from these
[turning points]. . . . Not that a furious stream of water thrust down,
when reaching the center point (which they themselves call incorpo- page 306
real 55 ), should stand suspended, go around in circles, swinging with
an incessant and perpetual swing? 56
“Each body is guided by motion according to nature, if it is not turned
aside by something else.” To clarify the meaning of this we must first of all
ask what is meant by “according to nature” ( ©). To Aristotle, who
discussed the question at length, in the De caelo in particular, the answer
depended on the body’s nature: for heavy bodies, motion according to
sources, explicitly makes the link between acceleration and decrease in cross-section, concluding
with “and so it breaks up [into drops]” (Jordanus Nemorarius, Liber de ratione ponderis, proposition
R4.16, in [Moody, Clagett], pp. 224–227). Finally, the relationship established by Strato between
drop formation and acceleration seems hard to explain in the absence of the theoretical notions
just discussed, as shown by the modern historians of philosophy who did not succeed in grasping
Strato’s argument (see, for example, [Rodier], p. 64, note 2).
53 The ideas discussed in the remainder of this section first appeared in [Russo: Plutarco], where
they are discussed at greater length. As fully explained there (p. 81), we adhere to the text as
preserved in the manuscripts at some of the spots where editors and translators have emended it.
54 Plutarch, De facie quae in orbe lunae apparet, 923C–D.
55 The claim that it is absurd that an incorporeal “point” should influence material bodies is
typical of the Skeptical critique of scientific theories. See, for example, Diogenes Laertius, Vitae
philosophorum, IX, 99, or the passage of Sextus Empiricus cited on page 202 (Adversus mathematicos,
I, 28).
56 Plutarch, De facie. . . , 923F–924C.
Revision: 1.11 Date: 2003/01/06 02:20:46