This question is more general than the problem of memory. Many different forms of words may be used to express the same “meaning”, and there seems no reason in mere habit to account for the fact that we sometimes use one form of words and sometimes another when we “think” of that which all the various forms of words express. The association seems to go, not direct from stimulus to words, but from stimulus to “meaning” and thence to words expressing the “meaning”. You may, for instance, be quite unable to recollect whether you were told “Jacob is older than Joseph”, or “Joseph is younger than Jacob”, though you may remember quite definitely that you were told the fact which both these forms of words express. Again, if you are learning, say, a proof of a mathematical theorem, you do not learn by heart what the book says, unless you are a very bad mathematician; you learn, as people say, to “understand” the proof, and then you can reproduce it in symbols quite different from those in the book. It is such facts, among others, that make it difficult to explain the mechanism of association, whether in memory or in “thought” in general, if we assume that words, or even sentences, are the terms associated. Perhaps, however, the theory as to the “meaning” of words which we developed in an earlier chapter may help us out of the difficulty. We defined the “meaning” of a word by means of its associations; therefore, if two words are synonyms, they have the same associations; and any stimulus which calls up one may also call up the other. The question which of two synonyms we use will then depend upon some extraneous circumstance.