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.

AT THE MERCY OF OUR ASSOCIATIONS.--Through the law thus operating we are
in a sense at the mercy of our associations, which may be bad as well as
good. We may form certain lines of interest to guide our thought, and
attention may in some degree direct it, but one's mental make-up is,
after all, largely dependent on the character of his associations. Evil
thoughts, evil memories, evil imaginations--these all come about through
the association of unworthy or impure images along with the good in our
stream of thought. We may try to forget the base deed and banish it
forever from our thinking, but lo! in an unguarded moment the nerve
current shoots into the old path, and the impure thought flashes into
the mind, unsought and unwelcomed. Every young man who thinks he must
indulge in a little sowing of wild oats before he settles down to a
correct life, and so deals in unworthy thoughts and deeds, is putting a
mortgage on his future; for he will find the inexorable machinery of his
nervous system grinding the hated images of such things back into his
mind as surely as the mill returns to the sack of the miller what he
feeds into the hopper. He may refuse to harbor these thoughts, but he
can no more hinder their seeking admission to his mind than he can
prevent the tramp fr