The corresponding facts may be expressed in psychological terms as follows: (1) Facts accustomed to being associated together in the mind have a tendency to reappear together. (2) The more frequently these facts appear together the stronger the tendency for the presence of one to insure the presence of the other. (3) The greater the tension, excitement or concentration when these facts appear in conjunction with each other, the more certain the presence of one is to cause the presence of the other. Several different types of association have been differentiated by psychologists from Aristotle down. It is to be kept in mind, however, that all association types _go back to the elementary law of habit-connections among the neurones_ for their explanation. ASSOCIATION BY CONTIGUITY.--The recurrence in our minds of many of the elements from our past experience is due to the fact that at some time, possibly at many times, the recurring facts were contiguous in consciousness with some other element or fact which happens now to be again present. All have had the experience of meeting some person whom we had not seen for several months or years, and having a whole series of supposedly forgotten incidents or events connected with our former associations flood into the mind. Things we did, topics we discussed, trips we took, games we played, now recur at the renewal of our acquaintance. For these are the things that were contiguous in our consciousness with our sense of the personality and appearance of our friend. And who has not in similar fashion had a whiff of perfume or the strains of a song recall to him his childhood days! Contiguity is again the explanation.