The doctrine of pores and effluvia, which formed so conspicuous an item in the physics of Empedokles, was applied by him to explain sensation. He maintained the general doctrine (which Parmenides had advanced before him, and which Plato retained after him), that sensation was produced by like acting upon like: Herakleitus before him, and Anaxagoras after him, held that it was produced by unlike acting upon unlike. Empedokles tried (what Parmenides had not tried) to apply his doctrine to the various senses separately.[128] Man was composed of the same four elements as the universe around him: and since like always tended towards like, so by each of the four elements within himself, he perceived and knew the like element without. Effluvia from all bodies entered his pores, wherever they found a suitable channel: hence he perceived and knew earth by earth, water by water, and so forth.[129] Empedokles, assuming perception and knowledge to be produced by such intercommunication of the four elements, believed that not man and animals only, but plants and other substances besides, perceived and knew in the same way. Everything possessed a certain measure of knowledge, though less in degree, than man, who was a more compound structure.[130] Perception and knowledge was more developed in different animals in proportion as their elementary composition was more mixed and varied. The blood, as the most compound portion of the whole body, was the principal seat of intelligence.[131] [Footnote 128: Theophrastus, De Sensu, s. 2, p. 647, Schneid.] [Footnote 129: Emp. Frag. Karst. v. 267, seq. [Greek: gnô=th', o(/ti pa/ntôn ei)si\n a)por)r(oai\ o(/ss' e)ge/nonto], &c. ib. v. 321: [Greek: gai/ê| me\n ga\r gai=an o)pô/pamen, u(/dati d' u(/dôr, ai)the/ri d' ai)the/ra di=on, a)ta\r puri\ pu=r a)i+\dêlon, storgê=| de\ storgê/n, nei=kos de/ te nei/kei+ lugrô=|]. Theophrastus, De Sensu, c. 10, p. 650, Schneid. Aristotle says that Empedokles regarded each of these six as a [Greek: psuchê\] (_soul_, _vital principle_) by itself. Sextus Empiricus treats Empedokles as considering each of the six to be a [Greek: kritê/rion a)lêthei/as] (Aristot. De Animâ, i. 2; Sext. Emp. adv. Mathem. vii. 116).] [Footnote 130: Emp. Fr. v. 313, Karst. ap. Sext. Empir. adv. Mathem. viii. 286; also apud Diogen. L. viii. 77. [Greek: pa/nta ga\r i)/sth' phro/nêsin e)/chein kai\ nô/matos ai)=san]. Stein gives (Emp. Fr. v. 222-231) several lines immediately preceding this from the treatise of Hippolytus; but they are sadly corrupt. Parmenides had held the same opinion before--[Greek: kai\ o(/lôs pa=n to\ o)\n e)/chein tina\ gnô=sin]--ap. Theophrast. De Sensu, s. 4.