scraping on the
sand as it neared the shore. And still others saw on the printed page
the words stating that Columbus discovered America in 1492. And so in an
infinite variety of images or ideas we may remember what we call the
same fact, though of course the fact is not really the same fact to any
two of us, nor to any one of us when it comes to us on different
occasions in different images.

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 mor