Since then to him the Platonic ideas were nothing apart from the individual objects, Aristotle could find no principle of movement or change in them; so he claimed that the doctrine of ideas was sterile and came to naught. In his own system he enumerated four principles or causes, which he insisted, however, are only known to us from individual things: the material cause (τὸ ἐξ oὗ γίνεταί τι, ἡ ὓλη), the formal cause (τὸ εἶδος, ἡ μορφή), the efficient cause (τὸ ὅθεν ἡ ἀρχή), and the final cause (τὸ oὗ ἕνεκα, τὸ τέλος). To make his meaning clear let us use in part his own illustrations: the material cause of the statue is the bronze of which it is made, just as in the example of the pot which we used a little while ago, the clay was its material cause; the formal cause is the idea of the statue, or of the pot, or of the octave in music, which the artist has in mind; the artist himself is the efficient cause; and the object of the action, the completed pot, statue, or whatever it may be, is the final cause.[215] That is to say the statue exists potentially in the bronze, the pot potentially in the clay, the octave potentially in the musical sounds, but these things can be called in actual existence only by the operation of the other three causes; and the same thing holds true in animate nature. It is possible therefore to state the matter generally and to say that in every case the individual is produced by the operation of the formal, efficient, and final causes on the material cause, bringing to actuality the potentiality in matter. Of course we may regard the formal, efficient, and final causes as different aspects of the same formal cause—a thing which Aristotle himself does in more than one passage,[216] so that in the last analysis he regards matter and form as the two causes or principles of things. These two are to him correlatives, each completing the other, although he gives greater importance to the formal than to the material cause. These two causes, he says, attract each other; and their union brings about movement, which is always the evolution of something from something else.