(_a_) It implies, of course, that _all_ the alternatives (before the mind) ‘work’ more or less. They must be (or appear) scientifically plausible, and proffer a more or less satisfactory explanation of some or all of the admitted ‘facts’. This is why agencies like the Devil, who could once be extensively alleged to explain anything unusual, have dropped out of the purview of science. (_b_) ‘Working’ must be conceived somewhat widely. Its _primary_ appeal is to the accepted principles and recognized interests of the science; practically to ‘work’ means to conduce to the development of the science on the recognized lines, and the proper judges of what ‘working’ counts are the experts who cultivate each science. (_c_) But there will often be complications due to certain disputable workings, of which the relevance is not yet established, and about these there will legitimately be differences of opinion. These should not be suppressed, but candidly argued out. (_d_) Moreover, every _new_ departure will be _pro tanto_ disputable, because it will conflict more or less with the vested interests of the established doctrines. One great factor in the ‘working’ of a new truth is the extent to which it upsets, or is thought to upset, the old, and demands a reconstruction of beliefs, a correction of authorities, a revision of text-books, a renewal of plant, &c. Hence what works best in the abstract may not do so under the actual conditions. It may ‘pay’ a professor better to be ‘orthodox’ than to be an innovator, and he is usually quite alive to this, though it does not render him a good investment scientifically for the institution that appoints him. If then we looked at this side of the matter alone, the verdict would always go against the novelty. For very few new truths are fortunate enough to find the field free and unoccupied. Usually they have to spring up in a soil densely overgrown with a rank growth of prejudices, dogmas, and superstitions, to which the world is accustomed and even devoted. So they have to fight for an opening in which they can take root and grow up.