The usual view of Nature assumes _two_ fundamentally different principles of motion, therefore it supposes that the movement of a body may have _two different origins_: _i.e._, that it proceeds either from the inside, in which case it is attributed to the _will_; or from the outside, and then it is occasioned by _causes_. This principle is generally taken for granted as a matter of course and only occasionally brought explicitly into prominence; nevertheless, in order to make the case quite certain, I will point out a few passages from the earliest to the latest authors in which it is specially stated. In Phædrus,[267] Plato makes the distinction between that which moves spontaneously from inside (_soul_) and that which receives movement only from outside (_body_)--τὸ ὑφ' ἑαυτοῦ κινούμενον καὶ τό, ᾧ ἔξωθεν τὸ κινεῖσθαι.[268]--Aristotle establishes the principle in precisely the same way: ἅπαν τὸ φερόμενον ἢ ὑφ' ἑαυτοῦ κινεῖται, ἢ ὐπ' ἄλλου (_quidquid fertur a se movetur, aut ab alio_).[269] He returns to the subject in the next Book, chap. 4 and 5, and connects it with some explanatory details which lead him into considerable perplexity, on account precisely of the fallacy of the antithesis.[270]--In more recent times again J. J. Rousseau brings forward the same antithesis with great _naïveté_ and candour in his famous "Profession de foi du vicaire Savoyard:"[271] "_J'aperçois dans les corps deux sortes de mouvement, savoir: mouvement communiqué et mouvement spontané ou volontaire: dans le premier la cause motrice est étrangère au corps mû; et dans le second elle est en lui-même._"--But even in our time and in the stilted, puffed-up style which is peculiar to it, Burdach holds forth as follows:[272] "The cause that determines a movement lies either inside or outside of that which moves. Matter is external existence; it has powers of motion, but it only brings them into play under certain spacial conditions and external oppositions: the soul alone is an ever active and internal thing, and only those bodies which have souls find within themselves inducement to move, and move of their own free will, independently of outer mechanical circumstances." [267] Plato, "Phæd." p. 319 Bip. [268] "That which is moved by itself and that which is moved from outside." [Tr.] And we find the same distinction again in the 10th Book "De Legibus," p. 85. [After him Cicero repeats it in the two last chapters of his "Somnium Scipionis." Add. to 3rd ed.] [269] "All that is moved, is moved either by itself or by something else." [Tr.] Aristotle, "Phys." vii. 2. [270] Maclaurin, too, in his account of Newton's discoveries, p. 102, lays down this principle as his starting-point. [Add. to 3rd ed.] [271] Émile, iv. p. 27. Bip. [272] Burdach, "Physiologie," vol. iv. p. 323.