1 The Birth of Science - MSRI

11.5 Two Modern Scientists: Kepler and Descartes 315
interaction between the sun and the planets was transmitted from ancient
to modern science thanks to the interest that scientists like Kepler had in
authors such as Pliny and Theon of Smyrna. Galileo’s rationalism had led page 388
him to reject as foolish the notion of gravitation, which in his time appeared
in works belonging to definitely unscientific traditions (Hermetic
and astrological texts, for instance), where it exemplified astral influences
and was yoked to religion and magic.
Of course some of Kepler’s sources were much more “scientific” than
the ones we recognize in the passages just quoted. He used Apollonius of
Perga and Pappus, and one of the classical works that drew him strongly
was Plutarch’s dialogue De facie quae in orbe lunae apparet, of which he
even published an annotated Latin translation. 95 One may ask whether
ancient sources helped Kepler in the long and arduous road that led him
to discover that planetary orbits are elliptic, as they helped him recognize
the motor role of the sun. The approach attested by the passages quoted
earlier, involving in particular a belief in the perfection of the spherical
shape, does not seem too likely to have led to the ellipticity of orbits on its
own, particularly since the observed data could be described equally well
through a system of epicycles.
To take another example of the importance of sources in determining
method, we can contrast Descartes’ Geometry with his The World, subtitled
Treatise on light. Let’s examine, in this second work, the passage about a
pebble revolving around in a sling: a subject of great interest, and one
about which, as far as we know, the only sources available were literary
and from the imperial age.
For example, suppose a stone is moving in a sling along the circle
marked AB [see figure], and consider it exactly as it is at the instant SL: supply figure
it arrives at the point A. You will readily find that it is in the process
of moving. . . toward C, for it is in that direction that its action is
directed in that instant. But nothing can be found here that makes its
motion circular. Thus, supposing that the stone then begins to leave
the sling and that God continues to preserve it as it is at that moment,
it is certain that He will not preserve it with the inclination to travel
in a circle along the line AB, but with the inclination to travel straight
ahead toward point C.
According to this rule, then, we must say that God alone is the author
of all the motions in the world in so far as they exist and in so far as
they are straight, but that it is the various dispositions of matter that
render the motions irregular and curved. Likewise, the theologians page 389
95 This appears in [Kepler: OO], vol. VIII.
Revision: 1.11 Date: 2003/01/06 07:48:20