As Aristotle has hitherto combated an external teleology, he directs another equally applicable remark (Phys. II. 9) against merely external necessity, and thus we come to the other side, or to how necessity exists in nature. He says in this regard: “Men fancy that necessity exists in this way in generation, just as if it were thought that a house existed from necessity, because heavy things were naturally carried downwards, and light things upwards, and that, therefore, the stones and foundation, on account of their weight, were under the earth, and the earth, because it was lighter, was further up, and the wood in the highest place because it is the lightest.” But Aristotle thus explains the facts of the case. “The house is certainly not made without these materials, but not on account of, or through them (unless the material so demands), but it is made for the sake of concealing and preserving certain things. The same takes place in everything which has an end in itself; for it is not without that which is necessary to its nature, and yet it is not on account of this, unless the matter so demands, but on account of an end. Hence the necessary is from hypothesis only, and not as end, for necessity is in matter, but end is in reason (λόγῳ). Thus it is clear that matter and its movement are necessity in natural things; both have to be set forth as principle, but end is the higher principle.” It undoubtedly requires necessity, but it retains it in its own power, does not allow it to give vent to itself, but controls external necessity. The principle of matter is thus turned into the truly active ground of end, which means the overthrow of necessity, so that that which is natural shall maintain itself in the end. Necessity is the objective manifestation of the action of its moments as separated, just as in chemistry the essential reality of both the extremes—the base and the acid—is the necessity of their relation.