No animal, therefore, is eternal.
But your school, Balbus, allows fire only to be the sole active principle; an opinion which I believe you derive from Heraclitus, whom some men understand in one sense, some in another: but since he seems unwilling to be understood, we will pass him by.

You Stoics, then, say that fire is the universal principle of all things; that all living bodies cease to live on the extinction of that heat; and that throughout all nature whatever is sensible of that heat lives and flourishes.

Now, I cannot conceive that bodies should perish for want of heat, rather than for want of moisture or air, especially as they even die through excess of heat; so that the life of animals does not depend more on fire than on the other elements.
However, air and water have this quality in common with fire and heat.
But let us see to what this tends.

If I am not mistaken, you believe that in all nature there is nothing but fire, which is self-animated.
Why fire rather than air, of which the life of animals consists, and which is called from thence _anima_,[248] the soul?
But how is it that you take it for granted that life is nothing but fire?
It seems more probable that it is a compound of fire and air.

But if fire is self-animated, unmixed with any other element, it must be sensitive, because it renders our bodies sensitive; and the same objection which I just now made will arise, that whatever is sensitive must necessarily be susceptible of pleasure and pain, and whatever is sensible of pain is likewise subject to the approach of death; therefore you cannot prove fire to be eternal.
You Stoics hold that all fire has need of nourishment, without which it cannot possibly subsist; that the sun, moon, and all the stars are fed either with fresh or salt waters; and the reason that Cleanthes gives why the sun is retrograde, and does not go beyond the tropics in the summer or winter, is that he may not be too far from his sustenance.
This I shall fully examine hereafter; but at present we may conclude that whatever may cease to be cannot of its own nature be eternal; that if fire wants sustenance, it will cease to be, and that, therefore, fire is not of its own nature eternal.
XV.