II. -- Physics.

25. General Principles. Matter and World-Soul. -- Under the title of Physics we may group all the studies relating to the manifestations of the Idea in the visible universe. Before examining the structure of the corporeal world and of man in particular, we must first find out the general relations of the phenomenal world to the world of Ideas. Visible things, the objects of opinion, are a partial and incomplete manifestation of the Ideas: for which latter Plato has jealously guarded the monopoly of reality. What is it that compels the Idea to come down from the "high estate" which it occupies in the absolute world, and to appear under shadowy and contingent forms? Or can it infold itself in ever-varying and perishable things without losing thereby, eo ipso, its unity and immutability? Plato does not concern himself with either of these difficulties; he assumes the fact of a reflection of the Idea in the sense-world; and he exerts all his efforts in explaining it. With a view to this, he appeals to matter and to world-soul.

Matter accounts for all nature's imperfections; these, as such, could not be ascribed by Plato to the Idea. While the Idea is reality, matter is non-being (me on).^{1} It is not a mass already formed, -- as one might be inclined to think from reading the poetical descriptions of the Timaeus -- but the indeterminate thing (apeiron), the "shapeless and invisible" element, the necessary condition for the visible materialization of the Idea. This receptacle in whose bosom are evolved all sense phenomena, is empty space, or place devoid of all content. While matter for Aristotle is that from which all sensible things are made, for Plato it is that in which they appear. In this way sensible things, the object of opinion, are a mixture (mikton) of being and non-being, a projection of the Idea into space. But space is only a condition for the appearance of the Idea. How is the latter reflected in phenomena? By the agency of the world-soul, is Plato's answer.

The soul of the world is the connecting-link between the Idea and matter. It is formed by the Demiurge of an alloy of two elements, the immutable and the mutable, which he calls the one and the other (tauton and thateron), probably the Idea and Matter^{2} (and cut through the centre into two parts that overlap each other surrounding the world). At once divisible and incorporeal and harmoniously constituted in geometrical^{3} proportions, it accounts for the beauty of the visible world and for the continuous conservation of its order. Evidently influenced by the constitution of man, Plato makes the world a huge animal (zôon) composed of a visible body and an invisible soul. The soul sets the colossal machine in motion, -- circular motion,^{4} which was considered by all antiquity as the most perfect of all motions. Finally, the soul of the world is endowed with knowledge, and the spherical movement by which it folds back on itself, as it were, and returns to the point from which it started, is at once the symbol and the sensible expression of conscious life.

It is an original and poetical conception, this theory of a world-soul; but it only emphasizes, without explaining, the initial affirmation of Plato's physics. It does not show the channel by which the Idea communicates itself to the phenomenon: the Idea and the phenomenon still stay side by side in an irreconcilable dualism.

{1} Historians are not agreed as to the proper interpretation of the term "matter" in Plato's philosophy.

{2} V. STÖCKL, tr. by Fr. FINLAY, S.J., p. 84.

{3} Mathematics holds an intermediate place between vulgar knowledge and philosophical knowledge (20).

{4} With this conception, Plato connects his system of astronomy.