ON THE NATURAL FACULTIES Book I

 [p. 93]kidneys, while the other approaches them but is certainly not inserted into them. Now, if the blood were destined to be purified by them as if they were sieves, the whole of it would have to fall into them, the thin part being and the thick part retained above. But, as a matter of fact, this is not so. For the kidneys lie on either side of the vena cava. They therefore do not act like sieves, filtering fluid sent to them by the vena cava, and themselves contributing no force. They obviously exert traction; for this is the only remaining alternative.

How, then, do they exert this traction? If, as Epicurus thinks, all attraction takes place by virtue of the rebounds and entanglements of atoms, it would be certainly better to maintain that the kidneys have no attractive action at all; for his theory, when examined, would be found as it stands to be much more ridiculous even than the theory of the lodestone, mentioned a little while ago. Attraction occurs in the way that Hippocrates laid down; this will be stated more clearly as the discussion proceeds; for the present our task is not to demonstrate this, but to point out that no other cause of the secretion of urine can be given except that of attraction by the kidneys,

We arrive at our belief by excluding other possibilities.

and that this attraction does not take place in the way imagined by people who do not allow Nature a faculty of her own.

i.e.the mechanistic physicists. cf. pp. 45-47.

For if it be granted that there is any attractive faculty at all in those things which are governed by Nature,

cf. p. 85, note 3.

a person who attempted to say anything else about the absorption of nutriment

The subject of anadosis is taken up in the next chapter. cf. also p. 62, note 1.

would be considered a fool.