Another important point in acquiring impressions in memory is this: _That the better the understanding of the subject or object, the clearer the impressions regarding it, and the clearer the recollection of it_. This fact is proved by experiment and experience. A subject which will be remembered only with difficulty under ordinary circumstances will be easily remembered if it is fully explained to the person, and accompanied by a few familiar illustrations or examples. It is very difficult to remember a meaningless string of words, while a sentence which conveys a clear meaning may be memorized easily. If we understand _what a thing is for_, its uses and employment, we remember it far more easily than if we lack this understanding. Elbringhaus, who conducted a number of experiments along this line, reports that he could memorize a stanza of poetry in about one tenth the time required to memorize the same amount of nonsense syllables. Gordy states that he once asked a capable student of the Johns Hopkins University to give him an account of a lecture to which he had just listened. "I cannot do it," replied the student; "it was not logical." The rule is: _The more one knows about a certain thing, the more easily is that thing remembered_. This is a point worth noting. CHAPTER VIII. Memory--Continued. The subject of memory cannot be touched upon intelligently without a consideration of the Law of Association, one of the important psychological principles. THE LAW OF ASSOCIATION. What is known in psychology as the Law of Association is based on the fact that _no idea exists in the mind except in association with other ideas_. This is not generally recognized, and the majority of persons will dispute the law at first thought. But the existence and appearance of ideas in the mind are governed by a mental law as invariable and constant as the physical law of gravitation. Every idea has associations with other ideas. Ideas travel in groups, and one group is associated with another group, and so on, until in the end every idea in one's mind is associated directly or indirectly with every other idea. Theoretically, at least, it would be possible to begin with one idea in the mind of a person, and then gradually unwind his entire stock of ideas like the yarn on the ball. Our thoughts proceed according to this law. We sit down in a "brown study" and proceed from one subject to another, until we are unable to remember any connection between the first thought and the last. But each step of the reverie was connected with the one preceding and the one succeeding it. It is interesting to trace back these connections. Poe based one of his celebrated detective stories on this law. The reverie may be broken into by a sudden impression from outside, and we will then proceed from that impression, connecting it with something else already in our experience, and starting a new chain of sequence.