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 pose. Next (5) the elaborate treatise ‘On the Generation of Animals’ worked out this subject, illustrating it with a wonderfully copious collection of facts, or supposed facts, and of the opinions of the day; and, lastly (6), the great treatise entitled ‘Researches about Animals,’ formed, as it were, the conclusion of the whole, by giving detailed observations upon many of the various living creatures which are the products of the working of nature’s general laws.

Aristotle justly drew a distinction between the way in which any phenomenon of nature would be considered and defined by a dialectician and by a physicist. Thus he says (‘On the Soul,’ I. i. 16): “Anger would be defined by a dialectician to be ‘a desire for retaliation,’ or something of the kind,—by a physical philosopher it would be defined as ‘a boiling up of the hot blood about the heart. It is needless to say that the Stagirite himself was great and unrivalled in his dialectical definitions,—those definitions which depended on grasping the essence of facts which are patent to all ages alike; while in his physical definitions, being destitute of facts which only later ages have brought to light, he was very imperfect and occasionally almost absurd. As a specimen of this we may mention his account of the vital principle or life, from the two points of view. He defines the vital principle (‘Soul,’ II. i. 6) to be “the essential actuality of an organism;” and this definition has met with high praise from modem physiologists, some of whom, indeed, appear simply to have repeated it in slightly different words. Thus Duges defines life as “the special activity of