These three modes of explanation (analysis, interpolation, subsumption) all consist in generalising or assimilating the phenomena.  The pressure of the air, of a liquid, and motion in the direction of least resistance, are all commoner facts than pumping; that light travels faster than sound is a commoner fact than a thunderstorm or gun-firing.  Each of the laws—­’Cats kill mice,’ ‘Mice destroy humble-bees’ nests,’ ’Humble-bees fructify red clover’—­is wider and expresses the resemblance of more numerous cases than the law that ’Clover depends on cats’; because each of them is less subject to further conditions.  Similarly, every step in the communication of thought by language is less conditional, and therefore more general, than the completion of the process.

In all the above cases, again, each law into which the phenomenon (whether pumping or conversation) is resolved, suggests a host of parallel cases:  as the modifying of air-waves by the larynx and lips suggests the various devices by which the strings and orifices of musical instruments modify the character of notes.

Subsumption consists entirely in proving the existence of an essential similarity between things where it was formerly not observed:  as that the gyrations of the moon, the fall of apples, and the flotation of bubbles are all examples of gravitation:  or that the purifying of the blood by breathing, the burning of a candle, and the rusting of iron are all cases of oxidation:  or that the colouring of the underside of a red-admiral’s wings, the spots of the giraffe, the shape and attitude of a stick-caterpillar, the immobility of a bird on its nest, and countless other cases, though superficially so different, agree in this, that they conceal and thereby protect the organism.

Not any sort of likeness, however, suffices for scientific explanation:  the only satisfactory explanation of concrete things or events, is to discover their likeness to others in respect of Causation.  Hence attempts to help the understanding by familiar comparisons are often worse than useless.  Any of the above examples will show that the first result of explanation is not to make a phenomenon seem familiar, but to put (as the saying is) ‘quite a new face upon it.’  When, indeed, we have thought it over in all its newly discovered relations, we feel more at home with it than ever; and this is one source of our satisfaction in explaining things; and hence to substitute immediate familiarisation for radical explanation, is the easily besetting sin of human understanding:  the most plausible of fallacies, the most attractive, the most difficult to avoid even when we are on our guard against it.