that both participate in investigation for Plato. There is no reason to suppose that Plato
thought natural science solely concerned with forms, and so entirely non-empirical, or
solely concerned with the physical and so unable to constitute knowledge.
Plato has been much criticized for appearing to denigrate the role of observation and
experiment in science. Plato has So ̄crate ̄s say (Republic 530b6–c1):
“It is by means of problems, then, that we shall proceed with astronomy as we do
geometry, and we shall leave the things in the heavens alone, if we propose by really
taking part in astronomy to make useful instead of useless the understanding that is
by nature in the soul.”The context and the conditional nature of this passage are critical here. Plato prescribes a
curriculum for the intellectual development of the guardians of his ideal state, not offering
a methodology for astronomy, nor does this passage have any implication for such a meth-
odology. It says that if we are to use astronomy to educate the guardians, then we use it
in this specific manner. The Timaeus (47b6–c5), more concerned with method, tells us in
contrast that:
“God devised and gave to us vision in order that we might observe the rational
revolutions of the heavens and use them against the revolutions of thought that
are in us, which are like them, though those are clear and ours confused, and
by learning thoroughly and partaking in calculations correct according to nature,
by imitation of the entirely unwandering revolutions of God we might stabilize the
wandering revolutions in ourselves.”If Plato’s Timaeus supports the idea that the motions of the Sun, Moon and planets can
be resolved into combinations of regular circular motions, then this is probably Plato’s most
important contribution to contemporary Hellenic science. While (apparent) motions of the
fixed stars were easy to model, motions of the other heavenly bodies were not. They were
commonly referred to as “wanderers,” as their motion appeared to defy simple laws.
The problem in ascribing regular, circular motion to Plato is that the astronomical model
of the Timaeus is very crude, using only two circular motions each for Sun, Moon and
five planets, and so can only reproduce very few phenomena. This appears to produce a
dilemma. Either Plato is ignorant of the phenomena, or his model must be able to account
for more of the phenomena, by using motions which are not regular and circular, if
Plato believes his model can reproduce all the phenomena of which he is aware. However,
S (in De Caelo = CAG 7 [1894] 504.17–20) tells us that authors proposed models
that could not account for all the phenomena of which they were aware. It may well be that
Plato considered his model of the Timaeus a prototype, not able to account for all the known
phenomena but showing the way in terms of regular circular motion. If so, Simplicius’
comment (in De Caelo = ibid., 488.18–20) makes sense:
“Plato posed the following problem for those engaged in these studies: ‘Which
hypotheses of regular and ordered motion are able to save the phenomena of the
planets?’”It matters little whether Plato or E, his associate, originated the idea as it is the
Timaeus which popularizes it. Eudoxos greatly improved on Plato’s model using a more
PLATO