This poetic description of the origin of the world is to be found in the "Timaeus." We have seen that the Ideas are absolute Being, and that things of sense are half real and half unreal. They are partly real because they participate in Being. They are partly unreal because they participate in not-being. There must be, therefore, a principle of absolute not-being. This, in Plato's opinion, is matter. Things of sense are copies of the Ideas fashioned out of, or stamped upon, matter. But Plato does not understand by matter what we, in modern times, understand by it. Matter, in our sense, is always some particular kind of matter. It is brass, or wood, or iron, or stone. It is matter which has determinate character and quality. But the possession of specific character means that it is matter with the copy of Ideas already stamped upon it. Since iron exists in great quantities in the world, and there is a common element in all the various pieces of iron, by virtue of which all are classed together, there must be a concept of iron. There is, therefore, an Idea of iron in the world of Ideas. And the iron which we find in the earth must be matter which is already formed into a copy of this Idea. It participates in the Idea of iron. The same remarks apply to any other particular kind of matter. In fact, all form, all the specific characters and {209} features of matter, as we know it, are due to the operation of the Ideas. Hence matter as it is in itself, before the image of the Ideas is stamped upon it, must be absolutely without quality, featureless, formless. But to be absolutely without any quality is to be simply nothing at all. This matter is, therefore, as Plato says, absolute not-being. Zeller conjectures, probably rightly, that what Plato meant was simply empty space. [Footnote 14] Empty space is an existent not-being, and it is totally indeterminate and formless. It accords with this view that Plato adopted the Pythagorean tenet that the differential qualities of material substances are due to their smallest particles being regular geometrical figures limited out of the unlimited, that is, out of space. Thus earth is composed of cubes. That is to say, empty space when bound into cubes (the limiting of the unlimited) becomes earth. The smallest particles of fire are _tetrahedra_, of air _octahedra_, of water _icosahedra_. [Footnote 14: _Plato and the Older Academy_, chap. vii. ]