Now as matter and form are the fundamental categories of Aristotle's philosophy, by means of which he seeks to explain the entire universe, it is essential that we should thoroughly understand their characteristics. {275} First of all, matter and form are inseparable. We think of them as separate in order to understand them clearly. And this is quite right, because they are opposite principles, and therefore they are separable in thought. But they are never separable in fact. There is no such thing as form without matter, or matter without form. Every existent thing, that is, every individual object, is a compound of matter and form. We may compare them in this respect to the material and the shape of a thing, though we must be careful not to think that form is merely shape. Geometry considers shapes as if they existed by themselves. But, in fact, we know that there are no such things as squares, circles, and triangles. There are only square objects, circular objects, etc. And as there are no shapes without objects, so there are no objects without shapes. We talk of things being "shapeless," but this only means that their shape is irregular or unusual. Some shape an object must have. Yet, though shape and matter are inseparable in fact, they are opposite principles, and are separable in thought. Geometry is quite right to treat shapes as if they existed by themselves, but it is nevertheless dealing with mere abstractions. Just in the same way, matter and form are never apart, and to think of form by itself or matter by itself is a mere abstraction. No such thing exists. In fact, to imagine that forms can exist by themselves was just the mistake of which, as we have seen, Aristotle accuses Plato. For the form is the Idea, and Plato imagined that Ideas exist in a world of their own.