Natural Knowledge in the Classical World 41four claims we saw earlier, that the cosmos is eternal, spherical, complete,
and divine? Context is everything.
SPHERES
It was common in antiquity, if not quite universal, to accept that the
cosmos was spherical. The roots of the idea go at least as far back as Par-
menides (fi fth century BCE), although it was the work of Aristotle that
gave it its most compelling formulations. Nevertheless, many of Aristotle’s
individual arguments, observations, and considerations—and above all
many of his questions—can be traced back to the greats among his pre-
decessors, Plato, Empedocles, Parmenides, and others. The perfection of
the sphere, its omnidirectional uniformity from the center, as well as its
ability to turn in place without projecting into the space around it, all had
for some time contributed to its plausibility as a model for the complete
perfection of the cosmos itself. These arguments in turn meshed very well
with the increasing amount of observational evidence accumulated over
the course of the fi fth and fourth centuries BCE about the behavior of the
stars and planets. Tantalizing bits of knowledge about the heavens had
been trickling in from Babylon during this period, stimulating and chal-
lenging homegrown Greek investigations (this Babylonian stream was to
become a fl ood in the next couple of centuries).^8 To the Greek mind, all
of it pointed with increasing force toward spherical rotation, such that
several astronomers before Aristotle then used spheres to model the mo-
tions of the heavenly bodies, including arguing for a fi nite sphere of the
heavens itself.
Enter Aristotle. His argument for the sphericity of the cosmos was com-
plex and multifaceted, extending (or being reworked) across several books.
Individual arguments in any one book are coherent and self- suffi cient, but
by the end of his oeuvre, a reader can look back and see the complex of
arguments that reinforce and interact with each other as well. In short,
there are a lot of good reasons for thinking of the cosmos as spherical,
with much cross- fertilization between them, and important consequences
extend out from the individual arguments and their combinations into
the history of later cosmology. One thing in particular worth paying at-
tention to is the complex interaction of observational and experiential
claims with highly developed (if often extremely unfamiliar) theory. Such
interaction and mutual reinforcement is a characteristic not just of an-
cient science, but of all science. The weirdness we see in Aristotle’s theo-