really know about
them. We see in animals, and to a lesser extent in plants, behaviour
more or less similar to that which, in us, is prompted by desires and
beliefs, and we find that, as we descend in the scale of evolution,
behaviour becomes simpler, more easily reducible to rule, more
scientifically analysable and predictable. And just because we are not
misled by familiarity we find it easier to be cautious in interpreting
behaviour when we are dealing with phenomena remote from those of our
own minds: Moreover, introspection, as psychoanalysis has demonstrated,
is extraordinarily fallible even in cases where we feel a high degree of
certainty. The net result seems to be that, though self-knowledge has
a definite and important contribution to make to psychology, it is
exceedingly misleading unless it is constantly checked and controlled
by the test of external observation, and by the theories which such
observation suggests when applied to animal behaviour. On the whole,
therefore, there is probably more to be learnt about human psychology
from animals than about animal psychology from human beings; but this
conclusion is one of degree, and must not be pressed beyond a point.
It is only bodily phenomena that can be directly observed in animals,
or even, strictly speaking, in other human beings. We can observe such
things as their movements, their physiological processes, and the sounds
they emit. Such things as desires and beliefs, which seem obvious
to introspection, are not visible directly to external observation.
Accordingly, if we begin our study of psychology by external
observation, we must not begin by assuming such things as desires and
beliefs, but only such things as external observation can reveal, which
will be characteristics of the movements and physiological processes of
animals. Some animals, for example, always run away from light and hide
themselves in dark places. If you pick up a mossy stone which is lightly
embedded in the earth, you will see a number of small animals scuttling
away from the unwonted daylight and seeking again the darkness of which
you have deprived them. Such animals are sensitive to light, in the
sense that their movements are affected by it; but it would be rash to
infer that they have sensations in any way analogous to our sensations
of sight. Such inferences, which go beyond the observable facts, are to
be avoided with the utmost care.
It is customary to divide human mov
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