So you must face the initial difficulty and overcome it. Indeed, =you must do more than merely understand=. The author’s undergraduates who break down in a preliminary examination always explain that they followed the lectures perfectly, and thought they understood the text-book, but that they were somehow unable to put things properly in their own words. The author’s small daughter who comes home with an elaborate example in compound interest explains, in the same manner, that she thoroughly understood the rule when She explained it, but that she can’t now see just how to go to work for herself. It may be that these excuses are not wholly reliable; they bear, at any rate, upon the present point. You must not only understand what you read as you read it; you must exercise your thought upon what you have read; you must be able to explain the paragraphs, in your own words, to others; you must find instances and illustrations for yourself; you must make the substance of the paragraphs a part of your habitual mental furniture; you must note how the old ways of thinking crop up to mislead you, and must correct and criticise the natural man. In a word, just as you practise your way into a language by reading, translating, writing, speaking; or just as you practise your way into algebra by doing exercise after exercise until the rule seems to be part of you and applies itself of its own accord; so must you =keep practising your psychology= until it becomes instinctive. You will gain some help by answering the appended questions; but after the book has done all that it can for you, the real induction into psychology remains to do for yourself. Some of the questions are concerned with forms of expression; and you should take these very seriously, since _language will be one of your greatest stumbling-blocks_. Language is older than science, and has developed under pressure of practical needs. Hence the phrases that come most naturally to your lips may embody a view of the world, or an attitude toward experience, that is totally foreign to the scientific context. If a visitor from Mars heard us all talking about the sunset, what would he think of our knowledge of the heavenly bodies? Yet we cannot escape from language; and if Newton could express his ideas in Latin, we ought to be able to express ours in English. It is a good plan, at the start, to have your technical definitions always at hand, and to try the effect of substituting these definitions for the words that you have been using; if the resulting clumsiness makes sense, you may let your first expressions pass; but if not, you should try again.