hat it rained on a day in the past. Without this
cortical activity, these facts would have existed just as truly, but
_you_ would never have known them. Without this neural activity in the
brain there is no consciousness, and to it we must look for the
recurrence in consciousness of remembered facts, as well as for those
which appear for the first time.

HOW WE REMEMBER.--Now, if we are to have a once-known fact repeated in
consciousness, or in other words _remembered_, what we must do on the
physiological side is to provide for a repetition of the neural activity
which was at first responsible for the fact's appearing in
consciousness. The mental accompaniment of the repeated activity _is the
memory_. Thus, as _memory is the approximate repetition of
once-experienced mental states or facts, together with the recognition
of their belonging to our past, so it is accomplished by an approximate
repetition of the once-performed neural process in the cortex which
originally accompanied these states or facts_.

The part played by the brain in memory makes it easy to understand why
we find it so impossible to memorize or to recall when the brain is
fatigued from long hours of work or lack of sleep. It also explains the
derangement in memory that often comes from an injury to the brain, or
from the toxins of alcohol, drugs or disease.

DEPENDENCE OF MEMORY ON BRAIN QUALITY.--Differences in memory ability,
while depending in part on the training memory receives, rest ultimately
on the memory-quality of the brain. James tells us that four distinct
types of brains may be distinguished, and he describes them as follows:

Brains that are:

(1) Like _marble_ to receive and like _marble_ to retain.
(2) Like _wax_ to receive and like _wax_ to retain.
(3) Like _marble_ to receive and like _wax_ to retain.
(4) Like _wax_ to receive and like _marble_ to retain.

The first type gives us those who memorize slowly and with much heroic
effort, but who keep well what they have committed. The second type
represents the ones who learn in a flash, who can cram up a lesson in a
few minutes, but who forget as easily and as quickly as they learn. The
third type characterizes the unfortunates who must labor hard and long
for what they memorize, only to see it quickly slipping from their
grasp. The fourth type is a rare boon to its possessor, enabling him
easily to stock his memory with valuable material, which is readily
availa