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 formulae 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.