6. Is the heaven composed exclusively of fire? Does the fire allow
any of its substance to flow off, or escape? Does it, therefore, need
being fed? (Plato[213]) thinks the body of the universe is composed of
earth and fire; fire to explain its being visible, and earth to explain
its being tangible. This would lead us to suppose that the stars are
composed of fire not exclusively, but predominatingly, since they seem
to possess a tangible element. This opinion is plausible because Plato
supports it with reasonable grounds. Sense, sight and touch would lead
us to believe that the greater part, if not the whole, of the heaven,
is fire. But reason suggests that the heaven also contains earth,
because without earth it could not be tangible.[214] This however does
not imply that it contains also air and water. It would seem absurd
to think that water could subsist in so great a fire; nor could air
survive therein without immediately being transformed to steam. It
might be objected that two solids which play the parts of extremes in
a proportion, cannot be united without two means.[213] This objection,
however, might have no cogency, for this mathematical relation might
not apply to natural things, as indeed we are led to surmise by the
possibility of mingling earth and water without any intermediary.
To this it may be answered that earth and water already contain the
other elements. Some persons might think that the latter could not
effectually unite earth and water; but this would not disturb our
contention that the earth and water are related because each of these
two elements contains all the others.