But the numerical prepossession--as we may call it--creates another assumption. Every number consists of units: every cube can be regarded as an aggregation of smaller cubes, and in measurement is (implicitly at least) so regarded. Transferring this to the physical world, every object is regarded as a composite--a Large, made up by the addition and juxtaposition of many (relatively) Littles. The essentials of the composite are here the elements that compose it: these, by a natural tendency, we proceed to conceive as remaining always unchanged, and giving rise by their peculiar juxtaposition to certain perceptions in the human being. You whirl rapidly a blazing piece of wood, and instead of a discontinuous series of flashes you see one orbit of luminous matter: or, let falling rain-drops take up a particular position in reference to your eyes and the sun, and a rainbow is visible. In both cases there is what may be called an illusion--the illusion, above all, of unity and continuity. Now what is in these cases obviously and demonstrably seen, is, as Leibniz in particular has reminded us, the general law of all matter as such. In the extended and material world there is nowhere a real unity discoverable. The small is made up of the smaller _ad infinitum_[1]. But the conclusion (which Leibniz drew)--that unity belongs only to v,' Monads and never by any possibility to a material substance, was not that commonly reached or accepted. There are--or there must be,--said the prevalent creed, ultimates, indivisibles, indecomposables, simples, atoms. These are the final bricks of reality, out of which the apparent universe is built: each with a maximum,--a _ne plus ultra_--of resistibility, hardness, fullness, and unsqueezable bulk. Into further details of these ultimate irreducibles we need not enter. It is sufficient to denote the general purport of the conception, and the tendency it implies.