1 The Birth of Science - MSRI

11.7 Newton’s Natural Philosophy 325
For each body is guided by motion according to nature, if it is not turned
aside by something else. 122
If the interpretation we gave to this passage in Section 10.3 is correct,
Newton, in the third and fourth definitions of the Principia, succeeded in
restoring the original meaning of the statement reported by Plutarch in
the italicized sentence. In choosing a name for the something else Newton
was more felicitous than Descartes: drawing from Aristotelian language,
he speaks of force. A little further in Plutarch’s dialog we read:
. . . the center. For this is [the point] toward which all weights, from
everywhere, are pressed, tending toward it and being moved toward
it and striving toward it. 123
The fifth definition in the Principia is virtually a translation of this. The
question arises: Were the definitions that Newton poses at the outset of
the foundational work of “modern science” influenced by Plutarch? The
evidence, though indirect, is quite strong.
Acquaintance with the De facie probably dated from Newton’s youthful
years, around 1664, when he started his scientific career precisely by
studying the appearance of the moon’s disk. We know that the De facie
played a role in guiding the formulation of the Principia, because Newton
included long excerpts thereof (including the ones we’ve quoted) in
the first draft of his book. They appear in the so-called Classical scholia,
notes about classical matters that Newton wrote for the Principia but did
not include in the published version. 126 And Newton opens his illustrative
scholium to the fifth definition (of centripetal force) with what may well
be an echo from the De facie:
Of this sort is gravity, by which bodies tend to the centre of the earth;
magnetism, by which iron tends to the loadstone; and that force,
whatever it is, by which the planets are continually drawn aside
from the rectilinear motions, which otherwise they would pursue,
and made to revolve in curvilinear orbits. A stone, whirled about in
a sling, endeavours to recede from the hand that turns it[.] 126a
With the loss of the original scientific treatises, Plutarch had become precious.
Copernicus, too, had resorted to him in reconstructing Aristarchus’
ideas (just as the inventors of the modern age had to start from Heron’s page 400
122
Plutarch, De facie quae in orbe lunae apparet, 923C–D.
123
Plutarch, De facie, 923E–F.
126
The Classical scholia were partly published for the first time in [Casini]. Newton’s personal
library naturally included Plutarch’s Opera Omnia (number 133 in [Harrison]).
126a
Newton, Principia mathematica, immediately after Definition V, Motte/Cajori translation.
Revision: 1.11 Date: 2003/01/06 07:48:20