e person who
is able to observe readily the plant and animal life as he walks through
the forest, possesses this skill, not because his physical eye, but
because his mind, has been prepared to see these objects. In other
words, it is because his knowledge is active along such lines that his
eye beholds these particular things. The chief reason, therefore, why
the exercise of any sense organ develops a power to perceive through
that sense, is that the exercise tends to develop in the individual the
knowledge and interest which will cause the mind to react easily and
effectively on that particular class of impressions. A sense may be
considered trained, therefore, to the extent to which the mind acquires
knowledge of, and interest in, the objective elements.

CHAPTER XXVI

MEMORY AND APPERCEPTION

=Nature of Memory.=--Mention has been made of the retentive power of the
nervous system, and of a consequent tendency for mental images to
revive, or _re-present_, themselves in consciousness. It must now be
noted that such a re-presentation of former experiences is frequently
accompanied with a distinct recognition that the present image or images
have a definite reference to past time. In other words, the present
mental fact is able to be placed in the midst of other events believed
to make up some portion of our past experience. Such an ideal revival of
a past experience, together with a recognition of the fact that it
formerly occurred within our experience, is known as an act of memory.

=Neural Conditions of Memory.=--When any experience is thus reproduced,
and recognized as a reproduction of a previous experience, there is
physiologically a transmission of nervous energy through the same brain
centres as were involved in the original experience. The mental
reproduction of any image is conditioned, therefore, by the physical
reproduction of a nervous impulse through a formerly established path.
That this is possible is owing to the susceptibility of nervous tissue
to take on habit, or to retain as permanent modifications, all
impressions received. From this it is evident that when we say we retain
certain facts in our mind, the statement is not in a sense true; for
there is no knowledge stored up in consciousness as so many ideas. The
statement is true, therefore, only in the sense that the mind is able
to bring into consciousness a former experience by reinstating the
necessary nervous impulses through the pro