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 Rh is presented under conditions that are in some respect unusual, it gives no jog to thinking, it makes no demand upon what is not present in order to be understood. And if the subject presented is totally strange, there is no basis upon which it may suggest anything serviceable for its comprehension. When a person first has to do with fractions, for example, they will be wholly baffling so far as they do not signify to him some relation that he has already mastered in dealing with whole numbers. When fractions have become thoroughly familiar, his perception of them acts simply as a signal to do certain things; they are a "substitute sign," to which he can react without thinking. (Ante, p. 178.) If, nevertheless, the situation as a whole presents something novel and hence uncertain, the entire response is not mechanical, because this mechanical operation is put to use in solving a problem. There is no end to this spiral process: foreign subject-matter transformed through thinking into a familiar possession becomes a resource for judging and assimilating additional foreign subject-matter.

The need for both imagination and observation in every mental enterprise illustrates another aspect of the same principle. Teachers who have tried object-lessons of the conventional type have usually found that when the lessons were new, pupils were attracted to them as a diversion, but as soon as they became matters of course they were as dull and wearisome as was ever the most mechanical study of mere symbols. Imagination could not play about the objects so as to enrich them. The feeling that instruction in "facts, facts" produces a narrow Gradgrind is justified not because facts in themselves are limiting, but because facts are dealt out