Often we fail to trace the associations governing our ideas, but the chain is there nevertheless. One may think of a past scene or experience without any apparent cause. A little thought will show that something seen, or a few notes of a song floating to the ears, or the fragrance of a flower, has supplied the connecting link between the past and the present. A suggestion of mignonette will recall some past event in which the perfume played a part; some one's handkerchief, perhaps, carried the same odor. Or an old familiar tune reminds one of some one, something, or some place in the past. A familiar feature in the countenance of a passer-by will start one thinking of some one else who had that kind of a mouth, that shaped nose, or that expression of the eye--and away he will be off in a sequence of remembered experiences. Often the starting idea, or the connecting links, may appear but dimly in consciousness; but rest assured they are always there. In fact, we frequently accept this law, unconsciously and without realizing its actual existence. For instance, one makes a remark, and at once we wonder, "How did he come to think of that?" and, if we are shrewd, we may discover what was in his mind before he spoke. There are two general classes of association of ideas in memory, viz.: (1) Association of contiguity, and (2) logical association. Association of contiguity is that form of association depending upon the previous association in time or space of ideas which have been impressed on the mind. For instance, if you met Mr. and Mrs. Wetterhorn and were introduced to them one after the other, thereafter you will naturally remember Mr. W. when you think of Mrs. W., and vice versa. You will naturally remember Napoleon when you think of Wellington, or Benedict Arnold when you think of Major André, for the same reason. You will also naturally remember _b_ and _c_ when you think of _a_. Likewise, you will think of abstract time when you think of abstract space, of thunder when you think of lightning, of colic when you recall green apples, of love making and moonlight nights when you think of college days. In the same way we remember things which occurred just before or just after the event in our mind at the moment; of things near in space to the thing of which we are thinking.