This is the main conception of Aristotelian Physics. Its further development concerns the conceptions of the different objects of nature, a material for speculative philosophy which we have spoken of above (pp. 153-155), and regarding which Aristotle puts before us reflections both difficult and profound. Thus he at first (Phys. III. 1-3) proceeds from this point to movement (κίνησις), and says that it is essential that a philosophy of nature should speak of it, but that it is difficult to grasp; in fact, it is one of the most difficult conceptions. Aristotle thus sets to work to understand movement in general, not merely in space and time, but also in its reality; and in this sense he calls it “the activity of an existent thing which is in capacity, so far as it is in capacity.” He explains this thus: “Brass is in capacity a statue; yet the motion to become a statue is not a motion of the brass so far as it is brass, but a motion of itself, as the capacity to become a statue. Hence this activity is an imperfect one (ἀτελής),” _i.e._ it has not its end within itself, “for mere capacity, whose activity is movement, is imperfect.” The absolute substance, the moving immovable, the existent ground of heaven which we saw as end, is, on the contrary, both activity itself and the content and object of activity. But Aristotle distinguishes from this what falls under the form of this opposition, “That moving is also moved which has movement as a capacity, and whose immobility is rest. That in which movement is present has immobility as rest; for activity in rest, as such, is movement.” That is to say, rest is capacity for motion. “Hence movement is the activity of that which is movable (κινητοῦ),[93] so far as it is movable; but this happens from the contact of that which is motive (κινητικοῦ), so that at the same time it is posited as passive likewise. But that which moves always introduces a certain form or end (εἶδος), either this particular thing (τόδε), or a quality or a quantity, which is the principle and cause of the motion when it moves; thus man, as he is in energy, makes man from man as he is in capacity. Thus, too, it is evident that movement is in the movable thing: for it is the activity of this, and is derived from that which is motive. The activity of that which is motive is likewise not different, for both are necessarily activity. It is motive because it has the capacity for being so; but it causes motion because it energizes. But it is the energetic of the moveable (ἔστιν ἐνεργητικὸν τοῦ κινετοῦ), so that there is one energy of both; just as the relation between one and two is the same as that between two and one, and there also is the same relation between acclivity and declivity, so the way from Thebes to Athens is the same as from Athens to Thebes. Activity and passivity are not originally (κυρίως) the same, but in what they are inherent, in motion, they are the same. In Being (τῷ εἶναι) they