thing that has gone before."[460] But if the phenomena do not resemble any thing that has gone before, how can there be "uniformity" in the succession of phenomena? 4. The uniformity of the _constitution_ of material nature, or the principle that the same substances must always have the same essential properties, is undoubtedly a self-evident and necessary truth, an _à priori_, rational intuition. It is simply a statement in concrete form of the principle or law of identity (A = A, or A is not equal to non-A). As we have already observed, a substance which ceases to have the same essential properties ceases to be the same substance; for substances are only known to us through their properties. But this "uniformity of co-existence" is distinct from "uniformity of succession," and we can not infer the latter from the former. Admitting that the same substance must always have the same properties, we can not affirm that the same substances will always be collocated in the same manner, or distributed in space with the same uniformity. In fact, "we can discover nothing regular in the distribution of matter through space; we can reduce it to no uniformity, to no law."[461] Matter is never replaced in its original position; "nothing repeats itself, because nothing can be placed in the same conditions; the past is irrevocable."[462] Even should we say with Sir William Thomson that "motion constitutes the very essence of what is commonly called matter," still we know with infallible certainty that there must be a _something_ that moves, and that this something which moves must have ultimately a definite _mass_ (inertia) and a measurable _velocity_, and that the _energy of motion_ to which the power of doing work is due is proportionate to the mass multiplied into the square of the velocity. Matter, then, is something more than motion.[463] We know further that there are different "modes of motion"--transitive, rotatory, vibratory, pulsatory, gyratory--and that these are undergoing perpetual transformation or conversion one into the other. And, finally, we know that the quantities of visible molar energy, and of invisible molecular energy (as heat, light, electricity, magnetism), are not uniform; on the contrary, the quantity of mechanical energy is being continually dissipated--that is, transformed into radiant heat, "which may be compared to the wasteheap of the universe,"[464] and uniformly diffused heat will not yield a single unit of work.