كتاب الحيوان (Book of Animals)

He says: Among the evidence that light is lighter than heat is that fire produces light at the height of its intensity, so its light reaches you while its heat does not. If a candle were in a room with a ceiling, the light would rise into the air until you found nothing of it on the ground except something faint, whereas the heat would remain in its initial state. Abu Ishaq said: The Daysaniyya a dualist sect claimed that the origin of the world is only from light and darkness, and that heat, cold, color, taste, sound, and smell are merely results determined by the extent of their mixture. It was said to them: We find that bread, when mixed with milk, becomes a grey body; and when you mix sabir aloe with honey, it becomes a bitter-tasting body. Whenever we add more of one of them, it gives us more of that taste in proportion to what we added. Likewise, we find all compounds. Why is it that when we mix two things from the category of sights, we end up with things of touch, taste, and smell? This same [objection] applies to those who claimed that all things were generated from those four things which are the portion of a single sense.

Abu Ishaq said: If some people claim that here is a sense which is a soul, and it is a fifth pillar, we do not disagree with them. But if they claim that things develop a sense when they are mixed in a certain way, how did the mixture come to produce a sense for them, when each one of them, if isolated, is not possessed of sense and is harmful to the body? If it exceeds [a certain proportion], it ruins its sense, and the rule for a small amount of that is not the same as the rule for a large amount. Why is it not permissible to combine light with light, so that it creates a prevention of perception? If they excuse themselves by citing mixture, ‘aqs knottedness/binding, and water, and say: "We find that each of these three is not black, but when they are mixed, they become a single body blacker than night, or obsidian, or a crow." (Abu Ishaq said): Between me and you regarding that is a difference. I claim that blackness can be latent and can be prevented from being seen; when its preventer vanishes, it appears, just as I say regarding fire, stone, and other latent things. If you say this, then you have abandoned your own statement; and if you refuse, then you must accept the statement.

Abu Ishaq said: Many of them also erred, for they claimed that the temperament of an old man is balgham phlegm. If his temperament were phlegm, and phlegm is soft, moist, and white, his old age would not be characterized by emaciation, his color by blackness, and his skin by contraction.