A second law governing reproduction may be called the set of the mind. When a railway train enters a large station, there are many paths over which it might pass; but its actual path depends on the position which was given to the switches immediately before the train’s arrival. In a similar manner the path taken by the mind depends on the set established just a few seconds or minutes before by the contents of the mind. If during a conversation in English a French word is unexpectedly pronounced by some one, the other people, though perfectly familiar with the French language, may fail to understand it. The French sounds are unexpected--the track is there, but the switch is not properly set--and consequently the sounds remain ineffective. A certain book seen on my desk calls up associated ideas very different from those which are produced when I see it in the bookstore. The same thought leads to one conclusion in the dark or in a dream, to another conclusion in daylight or in the waking state. Every student is familiar with the difficulty of becoming conscious of the right kind of ideas after having just gone from one recitation room to another. After a few minutes the new set of the mind is established, and the difficulty has disappeared.
Many other factors are to be mentioned as influencing the train of thought. During the last decades many experimental investigations have been devoted, with much success, to their exact determination. Numerous methods have been used, some being only slight modifications of the conditions under which ideas are reproduced in ordinary life, others being more artificial in order to yield answers to special questions to which the other methods are not applicable. The common involuntary reproduction of ideas by words or pictures shown has been used in order to determine how this reproduction varies with different individuals under different circumstances, how much time it requires, and so on. Voluntary reproduction of impressions that have just been made (as used in school in dictation) has been used by presenting, optically or through speech, words, syllables, numbers, or pictures and telling the subject to write down everything remembered. The quantity of the matter retained, and the number and kind of errors, then permit many important conclusions. Also whole poems or pieces of prose have been memorized, and answers have been found to questions as to the length of time necessary for such memorizing under different conditions, and the number of additional repetitions needed to make the material learned available again after a greater number of days or weeks. The acquisition of the vocabulary of a foreign language or of a set of historical dates has been developed into a special method of hitting or missing. The material to be learned has been presented in pairs, and the number of pairs has been counted of which one element causes the mental reproduction of the other. By all these methods psychologists have definitely secured many rules which had been derived from earlier, less reliable experiences. Many new facts have also been discovered. Let us give a brief account of the results of this work.