كتاب الحيوان (Book of Animals)

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all of that [to be] either harmful or beneficial, nourishing or killing, painful or delightful. The existence of earth being salty, fresh, foul, or pleasant is no more deserving of being a cause for the existence of dryness, cold, heat, and moisture than the existence of moisture, dryness, heat, and cold is a cause for the existence of color, taste, and smell. People have rushed upon these accompanying accidents and participating bodies as a single rush upon this adornment and form [that] the first and the last have cast. He said: So how did their statement fall upon the share of this condition alone, while we have not seen [anything] from moisture or from wetness that has a benefit or harm that it is unique in, apart from these things?

He said: "The air differs according to the factors [acting] upon it from below and from above, and from the bodies surrounding it and mixed with it." It is a thin body, and it is in that [state] constrained. It is hollow and quick to accept [influence], and despite its thinness, it accepts that constraint, like the action of the wind and the skin-bag, for they push it from its sides, and that is for the reason of constraint and its being cut off from its form. Air is not a body that is rising or descending, but it is a body by which dwellings and ascents are known. Things are three: a thing that rises in the air, a thing that descends in the air, and a thing [that exists] with the air. Just as the rising [thing] in it and the descending [thing] cannot be anything but opposites, they said: How, then, with [the air itself]? They would agree if a person sent from his hand, while he was at the bottom of the water, an inflated skin-bag, and the bag rose to push the wind that is inside it, no one would be able to say: "That air's nature is rising." Rather, he should say: "Its nature is to go toward its essence, and it does not reside in other than its essence." Unless he says: "Its nature is to rise in water, just as the nature of water is to descend in air, and just as water seeks the essence of water and air seeks the essence of air."

They said: Fire is many different species, and likewise that which rises. It is inevitable, since they are different, that some of them should be faster than others, or that some of them, when they exit from the world of air and reach an end where there is no path, remain above the other that rose with it. And if it finds a way, it does not remain upon it. Evidence for that is that we find light rising, and sound rising, and we find darkness remaining. Likewise cold and moisture. Since it is true that these species are different, and since [something] occurs in a direction, we know that the direction does not differ between the species, nor does it agree, and that which reconciles them is the difference of actions. The two parts cannot be in agreement unless their joy is the same. And when it reaches the limit, the connection of each one of them to its companion becomes like the connection [of one part]...