Thus, in another way, natural selection would
help to adjust our ideas to realities, and to exclude the possibility of
anything like a permanent common error.

Yet once more, according to Mr. Herbert Spencer, the tendency to
agreement between our ideas and the environment would be aided by what
he calls the direct process of adaptation. The exercise of a function
tends to the development of that function. Thus, our acts of perception
must become more exact by mere repetition. So, too, the representations
and concepts growing out of perceptions must tend to approximate to
external facts by the direct action of the environment on our physical
and psychical organism; for external relations which are permanent will,
in the long run, stamp themselves on our nervous and mental structure
more deeply and indelibly than relations which are variable and
accidental.

It would seem, from all this, that so long as we are keeping to the
scientific point of view, that is to say, taking for granted that there
is something objectively real answering to our perceptions and
conceptions, the question of the possibility of a universal or
(permanently) common illusion does not arise. Yet a little more
reflection will show us that it may arise in a way. So far as the
logical sufficiency of the social consensus or common belief is accepted
as scientifically proved, it is open to suspicion on strictly scientific
grounds. The evolutionist's proof involves one or two assumptions which
are not exactly true.

In the first place, it is not strictly correct to say that all illusion
involves a practical unfitness to circumstances. At the close of our
investigation of particular groups of illusion, for example, those of
perception and memory, it was pointed out that many of the errors
reviewed were practically harmless, being either momentary and
evanescent, or of such a character as not to lead to injurious action.
And now, by glancing back over the field of illusion as a whole, we may
see the same thing. The day-dreams in which some people are apt to
indulge respecting the remote future have little effect on their
conduct. So, too, a man's general view of the world is often unrelated
to his daily habits of life. It seems to matter exceedingly little, in
general, whether a person take up the geocentric or the heliocentric
conception of the cosmic structure, or even whether he adopt an
optimistic or pessimistic view of life and its capabilities.