and the state of the attention or interest of the observer is wholly different in the ordinary circumstances of life from what it is when the observer is being tested by the psychologist. It is entirely possible that when fifty little black squares irregularly pasted on a large sheet of white cardboard are exposed for five seconds to the gaze of A and B, A will make a much better guess than B at their number, owing to his ability to concentrate his attention or to make a swift calculation; and yet that if A and B were in a hall with fifty people in it, B would instinctively have a much better idea of the number of people in it than A, owing to a habit of being interested in the scenes of which he is a natural part and in their significance. Again A, fixing his attention on the end of a black pointer moving over the edge of a white dial, may be vastly better able than B to get that ratio of the consciously observed space to the consciously observed time which is the velocity Professor Münsterberg desired his students to measure in one of his experiments; and yet if A and B were walking casually along the street, B might be an incomparably more reliable witness on the question whether an automobile was or was not exceeding the legal speed limit. And this matter of the different distribution of interest in different circumstances is only one of a vast number of elements which go to making the psychologist's test highly precarious. You must catch a man "in his habit as he lives," you must follow him into all sorts of situations under all sorts of circumstances, internal and external, before you can decide what value to attach to his statement as to the facts that come into his ordinary experience of daily life. The man who may be too dull-witted to understand the psychologist's question, too lethargic to make a decent observation of what is put before him by his examiner, or too "rattled" to state correctly the result of that observation, may be a man who, as he goes about his work or chats with his fellows, misses nothing of the ordinary human occurrences that take place around him. And, on the other hand, the man of quick intelligence and keen activity who, upon demand, can bring all his faculties to bear upon a subject on which he is challenged to make a creditable report may, not only in spite of having this temperament, but actually because of it, be the very man who habitually takes extremely imperfect notice of the visible and audible things that are going on around him all the time and that have for him no significance.
Difficulties like these—I do not say insuperable difficulties, but certainly difficulties that offer enormous resistance to the investigator—are inherent in the subject. But over and above these inherent difficulties are those which attach not so much to the investigation as to the investigator. As a practical proposition, Professor Münsterberg's project must contemplate the employment of the psychological expert as an expert, strictly speaking. His report on the capacity of a witness