Other Memory Material.—But sensory images are not the only material with which memory has to deal. We may also recall the bare fact that it rained a week ago today without having images of the rain. We may recall that Columbus discovered America in 1492 without visual or other images of the event. As a matter of fact we do constantly recall many facts of abstract nature, such as mathematical or scientific formulæ with no imagery other than that of the words or symbols, if indeed these be present. Memory may therefore use as its stuff not only images, but also a wide range of facts, ideas and meanings of all sorts.
4. LAWS UNDERLYING MEMORY
The development of a good memory depends in no small degree on the closeness with which we follow certain well-demonstrated laws.
The Law of Association.—The law of association, as we have already seen, is fundamental. Upon it the whole structure of memory depends. Stating this law in neural terms we may say: Brain areas which are active together at the same time tend to establish associative paths, so that when one of them is again active the other is also brought into activity. Expressing the same truth in mental terms: If two facts or experiences occur together in consciousness, and one of them is later recalled, it tends to cause the other to appear also.
The Law of Repetition.—The law of repetition is but a restatement of the law of habit, and may be formulated as follows: The more frequently a certain cortical activity occurs, the more easily is its repetition brought about. Stating this law in mental terms we may say: The more often a fact is recalled in consciousness the easier and more certain the recall becomes. It is upon the law of repetition that reviews and drills to fix things in the memory are based.
The Law of Recency.—We may state the law of recency in physiological terms as follows: The more recently brain centers have been employed in a certain activity, the more easily are they thrown into the same activity. This, on the mental side, means: The more recently any facts have been present in consciousness the more easily are they recalled. It is in obedience to this law that we want to rehearse a difficult lesson just before the recitation hour, or cram immediately before an examination. The working of this law also explains the tendency of all memories to fade out as the years pass by.
The Law of Vividness.—The law of vividness is of primary importance in memorizing. On the physical side it may be expressed as follows: The higher the tension or the more intense the activity of neural centers the more easily the activity is repeated. The counterpart of this law in mental terms is: The higher the degree of attention or concentration when the fact is registered the more certain it is of recall. Better far one impression of a high degree of vividness than several repetitions with the attention wandering or the brain too fatigued to respond. Not drill alone, but drill with concentration, is necessary to sure memory,—in proof of which witness the futile results on the part of the small boy who "studies his spelling lesson over fifteen times," the while he is at the same time counting his marbles.
5. RULES FOR USING THE MEMORY
Much careful and fruitful experimentation in the field of memory has taken place in recent years. The scientists are now able to give us certain simple rules which we can employ in using our memories, even if we lack the time or opportunity to follow all their technical discussions.
Wholes Versus Parts.—Probably most people in setting to work to commit to memory a poem, oration, or other such material, have a tendency to learn it first by stanzas or sections and then put the parts together to form the whole. Many tests, however, have shown this to be a less effective method than to go over the whole poem or oration time after time, finally giving special attention to any particularly difficult places. The only exception to this rule would seem to be in the case of very long productions, which may be broken up into sections of reasonable length. The method of committing by wholes instead of parts not only economizes time and effort in the learning, but also gives a better sense of unity and meaning to the matter memorized.