And just as our theories, as representative of actuality, fall before his unequalled grasp of fact, so the doctrine of ideas fell. It is not an adequate account of existence, as Plato himself shows in his “Parmenides”; it only explains things by putting their doubles beside them. For his own part Aristotle invented a great marching definition which, with a kind of power of its own, cleaves its way through phenomena to limiting conceptions on either hand, towards whose existence all experience points. In Aristotle’s definition of matter and form as the constituent of reality, as in Plato’s mystical vision of the kingdom of ideas, the existence of the higher dimensionality is implicitly involved. Substance according to Aristotle is relative, not absolute. In everything that is there is the matter of which it is composed, the form which it exhibits; but these are indissolubly connected, and neither can be thought without the other. The blocks of stone out of which a house is built are the material for the builder; but, as regards the quarrymen, they are the matter of the rocks with the form he has imposed on them. Words are the final product of the grammarian, but the mere matter of the orator or poet. The atom is, with us, that out of which chemical substances are built up, but looked at from another point of view is the result of complex processes. Nowhere do we find finality. The matter in one sphere is the matter, plus form, of another sphere of thought. Making an obvious application to geometry, plane figures exist as the limitation of different portions of the plane by one another. In the bounding lines the separated matter of the plane shows its determination into form. And as the plane is the matter relatively to determinations in the plane, so the plane itself exists in virtue of the determination of space. A plane is that wherein formless space has form superimposed on it, and gives an actuality of real relations. We cannot refuse to carry this process of reasoning a step farther back, and say that space itself is that which gives form to higher space. As a line is the determination of a plane, and a plane of a solid, so solid space itself is the determination of a higher space. As a line by itself is inconceivable without that plane which it separates, so the plane is inconceivable without the solids which it limits on either hand. And so space itself cannot be positively defined. It is the negation of the possibility of movement in more than three dimensions. The conception of space demands that of a higher space. As a surface is thin and unsubstantial without the substance of which it is the surface, so matter itself is thin without the higher matter.