In mastering Aristotle’s cosmology, we have gained the key to his entire method of systematisation. Henceforth, the Stagirite has no secrets from us. Where we were formerly content to show that he erred, we can now show why he erred; by generalising his principles of arrangement, we can exhibit them still more clearly in their conflict with modern thought. The method, then, pursued by Aristotle is to divide his subject into two more or less unequal masses, one of which is supposed to be governed by necessary principles, admitting of certain demonstration; while the other is irregular, and can only be studied according to the rules of probable evidence. The parts of the one are homogeneous and concentrically disposed, the movements of each being controlled by that immediately outside and above it. The parts of the other are heterogeneous and distributed among a number of antithetical pairs, between whose members there is, or ought to be, a general equilibrium preserved, the whole system having a common centre which either oscillates from one extreme to another, or holds the balance between them. The second system is enclosed within the first, and is altogether dependent on it for the impulses determining its processes of metamorphosis and equilibration. Where the internal adjustments of a system to itself or of one system to the other are not consciously made, Aristotle calls them Nature. They are always adapted to secure its everlasting continuance either in an individual or a specific form. Actuality belongs more particularly to the first sphere, and possibility to the second, but both are, to a certain extent, represented in each.