There is another phase of thinking, which while chiefly cautionary, is also in part constructive. We have so often been warned to “avoid the treachery of words” and to “define all our terms” that a repetition of the advice seems unnecessary. But we cannot overlook the excellent counsel of Blaise Pascal. He urges that we not only define our terms, but that whenever we use them we mentally substitute the definition. However, this needs to be qualified. If every time we used a term we stopped to substitute its definition, our thought might be exact but would hardly move forward very rapidly. It will usually be sufficient simply to substitute the definition a few times, for after doing this we shall gradually come to know exactly what we mean by a term, and further substitution would merely waste time. Of course, all this need be applied only to terms new, technical or equivocal; or those used in a mooted proposition. I have spoken of analogy as a constructive method. This, however, should be used only for suggestion, for it is most dangerous. Often we use an analogy and are quite unaware of it. Thus many social and political thinkers have called society an “organism,” and have proceeded to deal with it as if it were a large animal. They have thought not in terms of the actual phenomena under consideration, but in terms of the analogy. In so far as the terms of the analogy were more concrete than those of the phenomena, their thinking has been made easier. But no analogy will ever hold good throughout, and consequently these thinkers have often fallen into error. The quickest way to detect error in analogy is to carry it out as far as it will go—and further. Every analogy will break down somewhere. Any analogy if carried out far enough becomes absurd. We are most likely to err when we carry an analogy too far, but not to the point where the absurdity is apparent. Take the analogy employed in our first chapter, comparing thinking and a ship. For the sake of the image I shall make this a motor-boat. We might carry this out further. We might compare the effect on the mind of books and experience to the fuel used for the engine. The brain, transforming outward experience into thought, might be paralleled with a carburetor transforming fuel into usable form. An idea may be compared to a spark. All this is very fascinating. It may even lead to suggestions of real value. But it is bound soon or late to develop into the ludicrous. The analogy in question, however, does not need to be developed to be confuted. For unless a boat has a propeller and a rudder, its engine is useless. A mind is capable of attaining truth without even being aware of the existence of a science of thinking or of logic.