Anaxagoras is cited by Aristotle as believing that "plants are animals and feel pleasure and pain, inferring this because they shed their leaves and let them grow again." The idea is fanciful, yet it suggests again a truly philosophical conception of the unity of nature. The man who could conceive that idea was but little hampered by traditional conceptions. He was exercising a rare combination of the rigidly scientific spirit with the poetical imagination. He who possesses these gifts is sure not to stop in his questionings of nature until he has found some thinkable explanation of the character of matter itself. Anaxagoras found such an explanation, and, as good luck would have it, that explanation has been preserved. Let us examine his reasoning in some detail. We have already referred to the claim alleged to have been made by Anaxagoras that snow is not really white, but black. The philosopher explained his paradox, we are told, by asserting that snow is really water, and that water is dark, when viewed under proper conditions--as at the bottom of a well. That idea contains the germ of the Clazomenaean philosopher's conception of the nature of matter. Indeed, it is not unlikely that this theory of matter grew out of his observation of the changing forms of water. He seems clearly to have grasped the idea that snow on the one hand, and vapor on the other, are of the same intimate substance as the water from which they are derived and into which they may be again transformed. The fact that steam and snow can be changed back into water, and by simple manipulation cannot be changed into any other substance, finds, as we now believe, its true explanation in the fact that the molecular structure, as we phrase it--that is to say, the ultimate particle of which water is composed, is not changed, and this is precisely the explanation which Anaxagoras gave of the same phenomena. For him the unit particle of water constituted an elementary body, uncreated, unchangeable, indestructible. This particle, in association with like particles, constitutes the substance which we call water. The same particle in association with particles unlike itself, might produce totally different substances--as, for example, when water is taken up by the roots of a plant and becomes, seemingly, a part of the substance of the plant. But whatever the changed association, so Anaxagoras reasoned, the ultimate particle of water remains a particle of water still. And what was true of water was true also, so he conceived, of every other substance. Gold, silver, iron, earth, and the various vegetables and animal tissues--in short, each and every one of all the different substances with which experience makes us familiar, is made up of unit particles which maintain their integrity in whatever combination they may be associated. This implies, obviously, a multitude of primordial particles, each one having an individuality of