§ 5. We may take a further most important step forward. In the all-embracing systematic whole the unity and the multiplicity must be equally real and each must be real through the other. How is this possible? Only on condition that the whole system forms a single experience, and that the constituent factors again are single experiences. This will perhaps be best brought out by examining some typical case of the kind of unity in multiplicity which is insufficient for our purpose. (_a_) The unity of the world cannot be that of a mere collection or aggregate. In a mere aggregate the elements are real independently of their relation to one another as elements in this aggregate. So long as we keep strictly to the case of what is no more than an aggregate, the quality of the elements is entirely unaffected by their inclusion in the aggregate. The aggregate has no unitary character of its own which reveals itself in and through the behaviour of its elements. Its unity consists in nothing more than the fact that we have found it convenient to think of its elements together. An aggregate of ten bricks, for instance, has no character as a whole beyond the mere fact of being thought of in one mental act. It has not even a collective weight until you put your ten bricks into the same cart, or on the same scale-pan, and then they have ceased to be a _mere_ aggregate in the very moment of exerting pressure upon the same surface, and have become a true material system. (_b_) Nor can the world of Reality be satisfactorily thought of as a mere _whole of parts_. A whole of parts approaches indeed more nearly to the ideal of a true systematic unity than a mere aggregate, inasmuch as it has a determinate single character as a whole, which manifests itself in the structure of the various parts. For this reason a geometrical figure or a machine is much more than a mere aggregate; it has a character as a whole, and this character is differently expressed by the construction of the different parts. The figure or machine is thus a true unity of differences. Yet in this case we cannot really say that the unity and the variety are equally real. For the whole cannot exist without the parts, whereas the parts may continue to exist, though not, of course, _as_ parts of this whole, without the whole. The whole is constituted by the successive generation or construction of the parts, and thus may be said to be formed out of pre-existing parts, and the parts again may survive the destruction of the whole. There is not that equal reality and complete mutual implication of the two sides which we have deemed necessary to a genuine systematic unity.