ble to him upon demand.

The particular type of brain we possess is given us through heredity,
and we can do little or nothing to change the type. Whatever our type of
brain, however, we can do much to improve our memory by obeying the laws
upon which all good memory depends.

2. THE FOUR FACTORS INVOLVED IN MEMORY

Nothing is more obvious than that memory cannot return to us what has
never been given into its keeping, what has not been retained, or what
for any reason cannot be recalled. Further, if the facts given back by
memory are not recognized as belonging to our past, memory would be
incomplete. Memory, therefore, involves the following four factors: (1)
_registration_, (2) _retention_, (3) _recall_, (4) _recognition_.

REGISTRATION.--By registration we mean the learning or committing of the
matter to be remembered. On the brain side this involves producing in
the appropriate neurones the activities which, when repeated again
later, cause the fact to be recalled. It is this process that
constitutes what we call "impressing the facts upon the brain."

Nothing is more fatal to good memory than partial or faulty
registration. A thing but half learned is sure to be forgotten. We
often stop in the mastery of a lesson just short of the full impression
needed for permanent retention and sure recall. We sometimes say to our
teachers, "I cannot remember," when, as a matter of fact, we have never
learned the thing we seek to recall.

RETENTION.--Retention, as we have already seen, resides primarily in the
brain. It is accomplished through the law of habit working in the
neurones of the cortex. Here, as elsewhere, habit makes an activity once
performed more easy of performance each succeeding time. Through this
law a neural activity once performed tends to be repeated; or, in other
words, a fact once known in consciousness tends to be remembered. That
so large a part of our past is lost in oblivion, and out of the reach of
our memory, is probably much more largely due to a failure to _recall_
than to _retain_. We say that we have forgotten a fact or a name which
we cannot recall, try as hard as we may; yet surely all have had the
experience of a long-striven-for fact suddenly appearing in our memory
when we had given it up and no longer had use for it. It was retained
all the time, else it never could have come back at all.

An aged man of my acquaintance lay on his deathbed. In his childhood he
had first learned t