Plutarch, De primo frigido
Goodwin, Ed.

Now that great rivers never freeze downwards is but consentaneous to reason. For those parts which are frozen above transmit no exhalation outward; for this, being penned up within and forced downward, affords heat to the moisture at the bottom. A clear demonstration of which is this, that when the ice is dissolved, you may observe a steam arising out of the water upwards in a very great quantity. And therefore the bodies of living creatures are warmest within in the winter, for that the heat is driven inward by the ambient cold. Now those upward exhalations and ascensions of the vapors deprive the waters not only of their heat but of their coolness. And therefore they that vehemently desire their drink to be cold never move the snow nor the moisture that is pressed out of it; for motion would deprive them both of the virtue which is required from them.

Now that this virtue is not the virtue of air, but of water, a man may collect by reasoning thus from the beginning. First, it is not probable that the air, which is next the sky, and touching the fiery substance is also touched by it, should be endued with a contrary virtue; for otherwise it is not possible that the extremities of the one should touch [p. 322] and be contiguous to the extremities of the other. Nor is it agreeable to reason that Nature should constitute that which is corrupted next in order to that which corrupts, as if she were not the author of community and harmony but of combat and contention. For she makes use of contrary things in sustaining the universe; but she does not use them pure and unmixed, nor so that they will be in hostility; but she uses such as have alternately a certain position and order which is not destructive, but which inclines them to communicate and co-operate one with another, and to effect a harmony between the opposing qualities. And this is the nature of the air, being expanded under the fire above the water, contingent and adhering to both, neither hot in itself nor cold, but containing an intermixture and communion of hot and cold, harmlessly intermixed in herself; and lightly cherishing the contrary extremities.