Let me begin by pointing out that, in the more modern conceptions of logic, it is recognised that there are no identically similar objective experiences; the disposition is to conceive all real objective being as individual and unique. This is not a singular eccentric idea of mine; it is one for which ample support is to be found in the writings of absolutely respectable contemporaries, who are quite untainted by association with fiction. It is now understood that conceivably only in the subjective world, and in theory and the imagination, do we deal with identically similar units, and with absolutely commensurable quantities. In the real world it is reasonable to suppose we deal at most with _practically_ similar units and _practically_ commensurable quantities. But there is a strong bias, a sort of labour-saving bias in the normal human mind to ignore this, and not only to speak but to think of a thousand bricks or a thousand sheep or a thousand sociologists as though they were all absolutely true to sample. If it is brought before a thinker for a moment that in any special case this is not so, he slips back to the old attitude as soon as his attention is withdrawn. This source of error has, for instance, caught nearly the whole race of chemists, with one or two distinguished exceptions, and _atoms_ and _ions_ and so forth of the same species are tacitly assumed to be similar one to another. Be it noted that, so far as the practical results of chemistry and physics go, it scarcely matters which assumption we adopt. For purposes of inquiry and discussion the incorrect one is infinitely more convenient.