It is plain, now, that these observations may be repeated. For one thing, there is a group of like cards in every class; and for another thing, the observer himself (since he works every day at the same time and under the same circumstances) is a fairly constant quantity. Besides, the observations may also be made by other observers, in other laboratories, under precisely the same circumstances; they may be repeated in just the same sense that a physical observation may be repeated. Secondly, the observations are isolated; they are made in a dark and quiet room, free from outside disturbance. No doubt, the observer’s thoughts may wander in the intervals between observations. For this reason, the experimenter gives a preconcerted signal, or calls out _Now_, a second or two before a word is shown; this signal warns the observer to pull himself together and to free himself from any such distractions. Thirdly, the observations are varied; for we employ all sorts of words, both normally printed and variously changed; and the stimuli may be presented for various lengths of time. Here, then, is a true psychological experiment; and if many observers, after many observations, give the same account of their perceptive experience, that account may stand as established psychological fact. Not all mental phenomena can be subjected to experiment so neatly as this particular perception; and the psychologist must still fall back, more often than he likes, upon casual observation or imperfect experiments. The reason is that _psychology has only recently become an experimental science_. Common-sense psychology is very old: we have a complete treatise in Greek from the hand of Aristotle, and a text-book in Pali compiled by some Buddhist sage, both dating from the fourth century B.C. But while it is in the sixteenth century of our era that the physicist abandons scholastic speculation and begins to study nature by experiment, it is not till the last quarter of the nineteenth that the psychologist follows suit. In or about the year 1875 the late Professor James, then instructor in anatomy and physiology at Harvard, had a single room devoted to psychological apparatus and experiments; and in 1879 Professor Wundt opened at the University of Leipsic, in a very modest way, the laboratory which has since become the most famous in the world. It is true that experiments in psychology had been made by individuals long before laboratories were thought of; but the same thing is true of physics and chemistry; and we may remember, when we come to the weak places of psychological exposition, that laboratory research and instruction are not yet fifty years old.