Empedocles indeed, as we learn from Aristotle’s second book, _De cœlo_ (c. 1, p. 284), had already grasped the conception of a tangential force arising from rotation, and counteracting gravity, which Aristotle again rejects. Quite the reverse, however, is Aristotle’s relation to the investigation of _organised_ nature. This is his field; here the wealth of his knowledge, the keenness of his observation, nay, sometimes the depth of his insight, astonish us. Thus, to give just one example, he already knew the antagonism in which in the ruminants the horns and the teeth of the upper jaw stand to each other, on account of which, therefore, the latter are wanting where the former are found, and conversely (_De partib. anim._, iii. 2). Hence then, also his correct estimation of final causes. Chapter XXVII. On Instinct And Mechanical Tendency. It is as if nature had wished, in the mechanical tendencies of animals, to give the investigator an illustrative commentary upon her works, according to final causes and the admirable design of her organised productions which is thereby introduced. For these mechanical tendencies show most clearly that creatures can work with the greatest decision and definiteness towards an end which they do not know, nay, of which they have no idea. Such, for instance, is the bird’s nest, the spider’s web, the ant‐lion’s pitfall, the ingenious bee‐hive, the marvellous termite dwelling, &c., at least for those individual animals that carry them out for the first time; for neither the form of the perfected work nor the use of it can be known to them. Precisely so, however, does _organising_ nature work; and therefore in the preceding chapter I gave the paradoxical explanation of the final cause, that it is a motive which acts without being known. And as in working from mechanical tendency that which is active is evidently and confessedly the _will_, so is it also really the will which is active in the working of organising nature.