he explains it on his doctrine of dualism between mind and matter; he teaches that sleep is produced by the reaction of the latter on the former. Even plants he regards as only rooted animals, motionless, but having sensations and desires; he imputes the superiority of man to the mere fact of his having hands. He explains our mental perceptions upon the hypothesis that we have naturally within us the contraries of all the qualities of external things; and that, when we consider an object, we become aware of the preponderance of those qualities in our mind which are deficient in it. Hence all sensation is attended with pain. His doctrine of the production of animals was founded on the action of the sunlight on the miry earth. The earth he places in the centre of the world, whither it was carried by a whirlwind, the pole being originally in the zenith; but, when animals issued from the mud, its position was changed by the Intellect, so that there might be suitable climates. In some particulars his crude guesses present amusing anticipations of subsequent discoveries. Thus he maintained that the moon has mountains, and valleys like the earth; that there have been grand epochs in the history of our globe, in which it has been successively modified by fire and water; that the hills of Lampsacus would one day be under the sea, if time did not too soon fail. [Sidenote: Doubts whether we have any criterion of truth.] As to the nature of human knowledge, Anaxagoras, asserted that by the Intellect alone do we become acquainted with the truth, the senses being altogether untrustworthy. He illustrated this by putting a drop of coloured liquid into a quantity of clear water, the eye being unable to recognize any change. Upon such principles also he asserted that snow is not white, but black, since it is composed of water, of which the colour is black; and hence he drew such conclusions as that "things are to each man according as they seem to him." It was doubtless the recognition of the unreliability of the senses that extorted from him the well-known complaint: "Nothing can be known; nothing can be learned; nothing can be certain; sense is limited; intellect is weak; life is short." [Sidenote: Anaxagoras is persecuted.]