Just as Aristotle invented that algebraical method of representing unknown quantities by mere symbols, not by lines necessarily determinate in length as was the habit of the Greek geometers, and so struck out the path towards those objectifications of thought which, like independent machines for reasoning, supply the mathematician with his analytical weapons, so in the formulation of the doctrine of matter and form, of potentiality and actuality, of the relativity of substance, he produced another kind of objectification of mind—a definition which had a vital force and an activity of its own. In none of his writings, as far as we know, did he carry it to its legitimate conclusion on the side of matter, but in the direction of the formal qualities he was led to his limiting conception of that existence of pure form which lies beyond all known determination of matter. The unmoved mover of all things is Aristotle’s highest principle. Towards it, to partake of its perfection all things move. The universe, according to Aristotle, is an active process—he does not adopt the illogical conception that it was once set in motion and has kept on ever since. There is room for activity, will, self-determination, in Aristotle’s system, and for the contingent and accidental as well. We do not follow him, because we are accustomed to find in nature infinite series, and do not feel obliged to pass on to a belief in the ultimate limits to which they seem to point. But apart from the pushing to the limit, as a relative principle this doctrine of Aristotle’s as to the relativity of substance is irrefragible in its logic. He was the first to show the necessity of that path of thought which when followed leads to a belief in a four-dimensional space. Antagonistic as he was to Plato in his conception of the practical relation of reason to the world of phenomena, yet in one point he coincided with him. And in this he showed the candour of his intellect. He was more anxious to lose nothing than to explain everything. And that wherein so many have detected an inconsistency, an inability to free himself from the school of Plato, appears to us in connection with our enquiry as an instance of the acuteness of his observation. For beyond all knowledge given by the senses Aristotle held that there is an active intelligence, a mind not the passive recipient of impressions from without, but an active and originative being, capable of grasping knowledge at first hand. In the active soul Aristotle recognised something in man not produced by his physical surroundings, something which creates, whose activity is a knowledge underived from sense. This, he says, is the immortal and undying being in man.