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 which they travel to us. Light, however, is an exception to this rule; it is an existence, not a motion; it produces alteration, and alteration of a whole mass may be instantaneous and simultaneous, as in a mass of water freezing. Thus Empedocles was mistaken (!) when he said that light travels from the sun to the earth, and that there is a moment when each ray is not yet seen, but is being borne midway.”—(‘Phys. Tracts.’ ‘On Sensation.’ vi,)

Among the permanent contributions to mental science which were made by Aristotle, none is more famous than his doctrine of the “Law of Association,” which he throws out while discussing Memory and Recollection in his ‘Physiological Tracts.’ He says, “Recollection is the recalling of knowledge. It implies the existence in the mind of certain starting-points, or clues, so that when you get hold of one you will be led to the rest. It depends on the law of association: we recollect when such and such a motion naturally follows such and such; we feel the latter motion, and that produces the former. In trying to recollect, we search after something that is in sequence, or similarity, or contrast, or proximity, to the thing which we want to recollect. Milk will suggest whiteness, whiteness the air, the air moisture, and this the rainy season, which was what we were trying to think of. No animal but man has the power of recollection, though many animals have memory. Re-