The nutrition and growth of animals is, of course, to be explained from the attraction of like to like. Each part of the body has pores into which the appropriate food will fit. Pleasure and pain were derived from the absence or presence of like elements, that is, of nourishment which would fit the pores. Tears and sweat arose from a disturbance which curdled the blood; they were, so to say, the whey of the blood.[633] Footnote 633: Nutrition, Aet. v. 27, 1; pleasure and pain, Aet. iv. 9, 15; v. 28, 1; tears and sweat, v. 22, 1. [Sidenote: Perception.] 118. For the theory of perception held by Empedokles we have the original words of Theophrastos:— Empedokles speaks in the same way of all the senses, and says that perception is due to the “effluences” fitting into the passages of each sense. And that is why one cannot judge the objects of another; for the passages of some of them are too wide and those of others too narrow for the sensible object, so that the latter either goes through without touching or cannot enter at all. R. P. 177 b. He tries, too, to explain the nature of sight. He says that the interior of the eye consists of fire, while round about it is earth and air,[634] through which its rarity enables the fire to pass like the light in lanterns (fr. 84). The passages of the fire and water are arranged alternately; through those of the fire we perceive light objects, through those of the water, dark; each class of objects fits into each class of passages, and the colours are carried to the sight by effluence. R. P. _ib._ But eyes are not all composed in the same way; some are composed of like elements and some of opposite; some have the fire in the centre and some on the outside. That is why some animals are keen-sighted by day and others by night. Those which have less fire are keen-sighted in the daytime, for the fire within is brought up to an equality by that without; those which have less of the opposite (_i.e._ water), by night, for then their deficiency is supplemented. But, in the opposite case, each will behave in the opposite manner. Those eyes in which fire predominates will be dazzled in the daytime, since the fire being still further increased will stop up and occupy the pores of the water. Those in which water predominates will, he says, suffer the same at night, for the fire will be obstructed by the water. And this goes on till the water is separated off by the air, for in each case it is the opposite which is a remedy. The best tempered and the most excellent vision is one composed of both in equal proportions. This is practically what he says about sight.