There can be no question that the concept of identity is a tool perfectly well adapted to deal approximately with nature in the region of our ordinary experience, but we have to ask a more serious question. Does not the apparent demand of our thinking apparatus to be furnished with discrete and identifiable things to think about impose a very essential restriction on any picture of the physical universe which we are able to form? We are continually surprising ourselves in the invention of discrete structure further and further down in the scale of things, whole sole _raison d'être_ is to be found entirely within ourselves. Thus Osborne Reynolds[12] has speculated seriously and most elaborately about an atomic structure in the ether, and we find Eddington[13] hinting at the existence of structure of an order of magnitude of 10^-40 cm. On a much larger scale of magnitude we also think in the same terms, and conceive positive and negative elementary charges with hard and impenetrable cores, which involves a complete change in the law of force at points sufficiently close. What physical assurance have we that an electron in jumping about in an atom preserves its identifiability in anything like the way that we suppose, or that the identity concept applies here at all? [Footnote 12: Osborne Reynolds, The Sub-Mechanics of the Universe, 254 pp., Cambridge University Press, 1903.] [Footnote 13: A. S. Eddington, Report on Gravitation, Lon. Phys. Soc., 1918, p. 91.] In fact, the identity concept seems to lose all meaning in terms of actual operations on this level of experience. The mind seems essentially incapable of dealing with continuity as a property of physical things; it is not even able to talk about continuity except in negative terms. To each attempted description of the properties of a truly continuous substance, it can say "No, it is not that", but cannot imagine experience which corresponds to what it conceives a really continuous thing ought to feel like. In terms of operations, continuity has only a sort of negative meaning. Now certain implications of this inability of the mind can be removed by appropriate postulates, as, for example, we can postulate the complete annihilation of a negative by a positive charge, as is now being done in certain speculations.[14] There is point in doing this, because the annihilation of two charges has physical meaning. But it is a question whether all implications of this habit of thought can be removed, and whether any picture that we can form of nature will not be tinged—sickbed o'er with the pale cast of thought. [Footnote 14: For example, J. H. Jeans, Nat. 114, 828-829, 1924.]