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HARVARD LAW REVIEW. since all sense-perception is based upon vibratory motions of matter, which are inconsistent with the hypothesis that all is static. In- deed, it could not be perceived even by an act of pure intelligence, for it is a violation of our hypothesis of unchangeability to sup- pose that in such a universe a perceiving mind could exist, since perception involves action of the perceived on the perceiver and reaction of the perceiver to the perceived, and action and- reaction involve change. Such a universe, then, might conceivably exist, but it could never be known to exist. Introduce change into such a universe, but change only. The resulting conception of a universe of constant change is not new ; it existed in the ancient notion that the universe is a mere fortui- tous concourse of atoms. Now this conception involves the notion of time, for change means a succession of events marked in time ; but this is the only addition to our prior conception. There is at any rate, so far as the hypothesis yet permits, no possibility of cause and effect. The impact of one body upon another would produce no change in either. Change might, or it might not, follow ; but, even if it did, by hypothesis it would not be the effect of the impact, and would not therefore be produced by it. Sen- suous perception of such a universe would be as impossible as in a static universe, for all forms of sensuous impression involve, not only vibration of matter, but also effects of vibration. That is, the vibrating matter must cause the perception in the perceiver ; but cause and effect are excluded from our hypothesis. By a parity of reasoning, intellectual perception is impossible, because percep- tion of all kinds involves the very causal relation which is rejected from our supposition. Like the purely static universe, therefore, a merely flowing universe might conceivably exist, but it could never be known to exist. From these considerations it follows that, in order that the external universe should be known at all as a self-subsisting reality, there must be something contained in it beyond the mere flux of hurrying atoms. There must be such a relation between the parts that one change necessarily produces another comparable to, and measured by, the former. A relation of this character we call a law, — a law of causality or of necessity. Such a law is insepa- rably involved in the act of knowing, and therefore knowledge of the mere existence of an external universe indubitably proves the existence of a law of necessary causality.