I
It has been remarked that man’s senses were given him, not to
philosophise with, but to help him in the struggle for existence;
Boltzmann, the great German physicist, was frankly distrustful of
many of the natural motions of the mind. He could admit that Science,
although often very abstract, had a certain validity, since it issues
in the prediction of events which are accessible to sense perception.
But philosophy, he insisted, was in an altogether different case, and
he thought the chances considerable that its impalpable conclusions
were the merest moonshine. It is a speculation that must have exercised
everyone who has whole-heartedly accepted the evolutionary account of
the rise of intelligence. Why should this instrument be adapted to
other than its original uses? Doubts of this kind, however, are both
too vague and too comprehensive to serve any useful purpose. They
do not tell us in what way and to what extent our intelligence is
untrustworthy; they do not enable us to make one step towards drawing
up an Index of Forbidden Subjects. At the most they enable a man with
a constitutional dislike of philosophic speculations to indulge his
contempt for that occupation with an easy conscience. Nevertheless,
a tincture of this doubt is very wholesome, and more particularly if
it be the result of an acquaintance with the history of human thought
rather than the product of a kind of lazy _a priori_ scepticism.
A student of the history of science, for instance, is inevitably led
to reflect on the curious nature of the barriers to further advance
which the mind itself has set up. It is as if the mind could only take
exercise within some imaginary prisoner’s yard, and that the great
advances were really the result of liberations. These liberations are
only partial; the mythical boundaries are set a little further off, but
it is agreed that the high walls exist.
It is interesting to review the progress of Science from this point of
view, to see it as a gradual secession from unwarrantable assumptions.
The exceedingly cautious, the almost groping character, of the
advance of knowledge, becomes very apparent. And, although such a
survey may lead us to become very conscious of this particular mental
limitation, we are not one whit nearer being enfranchised. It is still
the prerogative of genius to be innocent, to turn surprised eyes on
one of our most arbitrary assumptions, and to say: But that is not
necessary. The history of Astronomy, of course, provides some of the
best examples of mental prison yards. That the planets must move in
circles because the circle is the perfect figure is an assumption now
sufficiently remote from our acquired sense of probability to seem
exceedingly strange. That it was an assumption possessing a high degree
of obviousness is apparent from the fact that even Copernicus did not
question it. The attempt to enter into this assumption, to see it as
obviously reasonable, would be a useful exercise for the historian,
since it involves, very largely, a reconstitution of the mental life
of that age. It acquired its obvious character from the fact that it
_fitted in_; it was the natural companion of a great number of
other equally obvious assumptions; it was not an isolated eccentricity
of the mind. It is for that reason that Copernicus never freed himself
from it, and that Kepler only succeeded after a difficult struggle.
Kepler was required to question not merely an isolated doctrine, but to
escape from a veritable _Zeitgeist_. The Inquisitorial examination
of Galileo, also, was not directed merely to correcting the erroneous
statement of an isolated fact; it was, in truth, a whole system of
thought that stood on trial. It is this double aspect of any given
abandoned assumption that accounts for our unimaginative surprise on
learning that very intelligent men once mistook it for an obvious
truth. We are judging the assumption, not on its own merits, as it
were, but from the standpoint of an alien system of thought.
We can form a juster estimate of the degree of credulity manifested by
the contemporaries of Copernicus by considering assumptions that have
been but recently questioned, or rather, which have only recently been
generally questioned. The assumptions regarding animal psychology form
a vivid example. Such men as Darwin and Romanes found it quite natural
to assume that the emotions and many of the intellectual processes of
which they were conscious in themselves furnished an adequate key to
animal behaviour. It is an assumption which the average educated man
of to-day makes quite readily, although he may not share Aristotle’s
views on the perfection of circles. We now know that there is no reason
whatever to suppose, for example, that the psychology of snails has
the slightest resemblance to the psychology of human beings. We may be
confident that, in a very few years, the assumptions of Darwin and most
other people will appear almost inexplicably gratuitous. It will take
longer, we think, for the Freudian ideas about man himself to become
acclimatised; man will take a long time to learn that in trusting his
immediate awareness of himself he is making a number of unwarrantable
assumptions. The system of thought into which his present assumptions
fit is so profound and extensive that it is impossible, even now, to
picture the thoroughly enfranchised man.
A general acceptance of the Einsteinian ideas of space and time is
easier to predict. The current conceptions of space and time, although
Euclidean when reduced to a logical scheme, are not, in fact, present
as a logical scheme in the mind of the ordinary man. He is sufficiently
vague about his fundamental assumptions to offer no strenuous
resistance to their subtle modification. We think that part of his
general bewilderment about Einstein’s space and time is due to his
bewilderment on thinking about space and time at all. His assumptions
on these questions, whatever those assumptions may be, are not really
part of a general scheme of beliefs. Nothing that greatly concerns him
is incompatible with non-Euclidean geometry, and we confidently expect
that the grandchildren of the ordinary man will as blandly believe they
have swallowed Einstein as the contemporary ordinary man believes he
has swallowed Euclid. For an assumption which is not an integral part
of a general scheme of thought is readily abandoned. It is the lopping
of connections which the mind resists. It is no paradox to say that the
mathematician and philosopher finds it harder to accept Einstein than
does the ordinary man. That is because the mathematician’s acceptance
involves both believing more and disbelieving more.