But these ideas may be brought up not only voluntarily; we have also a certain degree of power in making these images clearer and more accurate. It is rather foolish to have the examiner invite the witness to “exert his memory, to give himself the trouble, etc.” This effects nothing, or something wrong. But if the examiner is willing to take the trouble, he may excite the imagination of the witness and give him the opportunity to exercise his power over the imagination. How this is done depends naturally upon the nature and education of the witness, but the judge may aid him just as the skilful teacher may aid the puzzled pupil to remember. When the pianist has completely forgotten a piece of music that he knew very well, two or three chords may lead him to explicate these chords forward or backward, and then--one step after another--he reproduces the whole piece. Of course the chords which are brought to the mind of the player must be properly chosen or the procedure is useless. There are rules for the selection of these clews. According to Ebbinghaus: “The difference in the content of the recollected is due to discoverable causes. Melodies may become painful because of their undesirable obstinacy in return. Forms and colors do not usually recur, and if they do, they do so with noticeable claims on distinctness and certainty. Past emotional conditions are reproduced only with effort, in comparatively pallid schemes, and often only by means of the accompanying movements.” We may follow these clews, in some directions at least, to our advantage. Of course, nobody will say that one should play tunes to witnesses in order to make them remember, because the tunes have sunk into the memory with such undesirable obstinacy as to be spurs to recollection. It is just as futile to operate with forms and colors, or to excite emotional conditions. But what has been said leads us back to the ancient rule of working so far as is possible with the constantly well-developed sense of location. Cicero already was aware of this: “Tanta vis admonitionis inest in locis, id quidem infinitum in hac urbe, quocumque enim ingredimur, in aliquam historiam vestigium possumus.” Indeed he deduces his whole doctrine of memory from the sense of location, or he at least justifies those who do so.