rstand what the
nature of man is, apart from its training and education during
the life of the individual, must start with the realization that
man is a human animal. As a human being he is strikingly
set off by his upright posture and his large and flexible hand.
But chiefly he is distinguished by his plastic brain, upon
which depends his capacity to perform the complex mental
activities--from administering a railroad to solving problems
in calculus--which constitute man's outstanding and
exclusive characteristic.[1]

[Footnote 1: The thinking process is discussed in detail in
chapters III and XIV.]

But in his structure and functions man bears, as is now well
known, a marked resemblance to the lower animals. His
respiratory and digestive organs, for example, may be duplicated
as far down in the animal scale as birds and chickens.[2]
Man's whole physical apparatus and mode of life, save in
complexity and refinement of operations, are the same as
those of any of the higher mammals. But more important for
the student of human behavior, man's mental life--that is,
his way of responding to and dealing with his environment--is
in large part identical with that of the lower animals,
especially of the most highly developed vertebrates, such as the
monkey. They have, up to a certain point, precisely the
same equipment for adjusting themselves to the conditions of
life. Apart from education, both man and animal are endowed
with a set of more or less fixed tendencies to respond in
specific ways to specific stimuli. These inborn or congenital
tendencies are generally known as reflexes or instincts.[1]
These are unlearned ways, exhibited by both human and
animal organisms, of responding promptly and precisely, and
in a comparatively changeless manner to a given stimulus
from the environment. These tendencies to act, while they
may be, and most frequently are of advantage to the organism,
are not conscious or acquired. They are irresistible
impulses to do just such-and-such particular things in
such-and-such particular ways when confronted with just
such-and-such particular situations. In the well-known words of
James:

[Footnote 2: With certain modifications accounted for in their historical
"descent" with modification from a common ancestor. See Scott:
_Theory of Evolution_.]

[Footnote 1: The difference between the two is largely one of complexity.
By a reflex is meant a very simple and comparatively rigid response; by