We may divide scientific theories into two classes, which have recently
been distinguished by Einstein as theories of construction and theories
of principle. His own theory of relativity is a theory of principle,
and its attraction resides in its logical perfection. Such theories,
whatever charm they may have for the logician, are not, man being
constituted as he is, felt to be sufficient. A principle which natural
phenomena obey, and which enables equations to be deduced expressing
the relations between phenomena, is, to a few austere souls, all with
which science need concern itself, but the majority of men require, in
addition, something they call an “explanation” of the relations deduced
from the principle. They desire to see events described in terms with
which they are familiar. Thus, a description of the behaviour of the
material universe in terms of the mutual impacts of little billiard
balls would afford genuine satisfaction to the mind, and important
advances have been made in science by the attempt to describe phenomena
in these terms. The assumptions which underlie some such attempts
may seem, to the logician, preposterous, but there is no doubt that
the mind is impelled to make such assumptions. Our familiarity with
the motions of matter in bulk makes it quite natural that we should
endeavour to give, as far as possible, dynamical explanations of
events, although, if we stop to ask ourselves why nature should be
flexible enough to admit of descriptions in such terms, we are at a
loss for an answer.
The history of theories of the æther is particularly instructive from
this point of view, because the irrational nature of the impulse is
here most clearly apparent. The attempt to explain phenomena in terms
of an æther has led to some very remarkable theories of the nature of
matter itself. It has been supposed, for instance, that the ultimate
particles of matter are vortical whirls in the æther, or, again, points
of a very special kind of strain in the æther. Nevertheless, a theory
of the æther is regarded as unsatisfactory which is not couched in
terms of the observed behaviour of ordinary matter as we know it. A
dynamical explanation is always sought after, and a great part of the
scientific effort of the nineteenth century was devoted to describing
the æther as an elastic solid. But men of science were not content
with showing that the laws of dynamics could be applied to the æther;
many of them endeavoured to devise models which should represent, on a
large scale, the actual construction of the æther. It is difficult to
know to what extent their authors supposed these models to correspond
to the reality; it is probably not sufficient, however, to say that
they regarded them merely as furnishing useful tools for subsequent
investigations. The models were usually extremely complicated, for,
from the very beginning, the æther proved somewhat recalcitrant to this
attempt to represent it as an elastic solid. The most obvious objection
to this representation was provided by the observed motions of the
planets. It could be proved that, if there were any resistance to their
motions round the sun, it must be excessively minute, and how was this
to be combined with the hypothesis that they were moving with great
speed through an elastic solid? The answer was found in cobbler’s wax.
Sir George Stokes noticed that cobbler’s wax, although rigid enough to
be capable of elastic vibration, is yet sufficiently plastic to permit
other bodies to pass slowly through it. We have only to imagine that
in the æther these qualities are much exaggerated, and the motion of
the planets presents no difficulty. If no substance like cobbler’s
wax happened to be known it is difficult to know what satisfactory
answer could be returned to the objection. Here we have the first
glimpse of the remarkable combination of qualities with which it was
found necessary to dower the æther. The mathematical examination of
the properties of the æther, undertaken by such men as Navier, Cauchy,
Poisson, Green, was continually leading to queer and unsatisfactory
results, unsatisfactory, that is, in the light of our experience of
the properties of matter. Cauchy, in particular, deduced a number of
remarkable physical properties which were irreconcilable with one
another, although one of his theories, that of the æther considered as
a kind of foam, attracted the attention of Lord Kelvin.
With the rise of Maxwell’s electromagnetic theory, the elastic solid
æther received less attention. Maxwell himself, in his great treatise,
gives no mechanical explanation of his theory; he merely shows that
an infinite number of mechanical explanations are possible. With the
publication of Einstein’s first principle of relativity in 1905,
however, the æther began to disappear; and now, with the generalised
theory of relativity, it has become a mere ghost. There are still
sturdy champions of the æther, and, indeed, it seems a pity to have
to abandon the mechanical explanations it promised. But possibly
the attempt to find dynamical explanations of this kind is doomed
to failure; perhaps, after all, nature is not flexible enough. The
orientation of modern science is in another direction. It is towards
a more abstract class of theories altogether--theories which tell us
nothing about the mechanism of a process, but tell us the principles
the process must obey. Such theories effect a vast unification of
knowledge. They are magnificently comprehensive, and it is possible
that they contain all that we can really know, although men will long
be reluctant to abandon all hope of ever approaching reality with the
intimacy that the theory of the æther seemed to promise.
V
Whether or not it be true that the proper study of mankind is man, it
is certain that he finds great difficulty in studying anything else.
His first impulse, when he thinks about the universe at large, is to
consider it in reference to himself, and to explain it in terms of his
own actions and desires. In Astronomy, for example, it long seemed
quite reasonable that in the peculiarities of men’s bodies should be
found the system on which the universe is constructed. The arguments
of Galileo’s contemporaries amuse us now, for we have learned modesty,
but the tendency to explain all things in purely human terms, as it
were, is by no means yet extinct, and is still a hindrance to science.
It is even hinted that man’s explanation of himself is not free from
bias; psychologists inform us that a man’s account of his own actions
is not always to be trusted, that the true springs of his conduct
are usually those he would blush to own. But if we are to say that
man’s speculations about the universe show an overwhelming sense of
his own importance we must allow him also a certain generosity. Until
quite recent times he was willing to dower almost anything, animate or
inanimate, with his own attributes. He credited stones with life and
trees with desire, while the whole animal world were his brothers. He
could admire the loving sentiments of the dove and weep for the sorrows
of the crab. A pathetic confidence in man as the type and exemplar
of the universe informed nearly all the early writings on animal
psychology, and Descartes’ theory that animals were automatic roused
a sentimental indignation which has not yet subsided. Nevertheless,
comparatively recent investigations tend to overthrow the natural
assumption that worms and insects are little men inhabiting strange
bodies. The modern biologist refuses to be conscience-stricken when
referred to the industry of the bee or the conjugal perfections of the
dove. It is only recently that he has become so heartless. Darwin,
in a celebrated passage, describes with simple reverence the mutual
affection existing between snails. The intelligence of these little
creatures was also estimated highly by Romanes. Loeb, the great
American biologist, did much to upset this naïve anthropomorphism.
He took some worms who are “always attracted by light,” and showed
that this movement did not testify to a “more light” cry in these
little souls, but was a purely automatic proceeding. The worm places
itself so that both sides of its body are equally illuminated. It is a
mechanical action due to the influence of light on the living matter of
its body. If there are two lights the worm passes between them, thus
securing equal illumination of its two sides.
The crab which, being held by a claw, sheds that claw and hurries to
the nearest rock for shelter, is found to do the same thing after its
eyes or brain have been destroyed. Dr. Georges Bohn, who has made many
experiments to determine how far the actions of the lower animals are
purely mechanical, gives an interesting account of a certain parasitic
worm which attaches itself to the fish called the torpedo. He finds
(1) that if the amount of salt in the water be varied the reactions
of the worm alter; (2) that if light be allowed to play first on one
part and then on another part of the worm, its reactions alter; (3) if
the animal has already taken up its position, attached to the glass,
for instance, and a shadow be passed over the top of the vessel, the
whole body of the worm turns itself into the vertical in such a way
that if the shadow were caused by a passing torpedo, the worm could
attach itself to the fish. If, however, it be already attached to a
torpedo, it does not raise itself at a passing shadow. Here, then, is
an _association_ between the region of the body excited by light
and the part fixed to the fish. It was found, also, that the crab which
abandons its claw only does so when held by a certain part. The action
appears to be purely automatic. If it were dependent in any way on the
crab’s simultaneous visual perceptions, for instance, an associative
phenomon would be established. But experimental tests find no such
correspondence. As the result of numerous experiments of this kind
biologists have become very wary of offering psychical explanations of
the actions of the lower animals. Even when genuine associations are
established one must be careful not to interpret them in terms of human
psychology. In the very description of experiments an unwarrantable
turn may be given to the phenomena by the fact that words of ordinary
language inevitably call up associations which may be out of place in
the discussion. To say that an amœba _learns_ to reject certain
foreign particles in a solution, for instance, is a statement that
requires careful interpretation. How are we to picture an amœba
_learning_ something?
But, indeed, the danger of anthropomorphic interpretations becomes
very obvious when we reflect on the purely physical phenomena which
accompany man’s own emotions. If the James-Lange theory be correct,
it is in terms of these physical phenomena that we must understand
man’s emotions. Now consider the example given in Washburn’s book,
_The Animal Mind_. An angry man has a quickened heartbeat, altered
breathing, a change in muscular tension, and a change in the blood.
Consider a wasp. It has no lungs, but breathes through its tracheæ;
the circulation of its blood is fundamentally different from that in
man; all its muscles are attached internally because its skeleton is
everywhere external. What, then, is an “angry” wasp? It seems clear
that if a man is to study anything but man he must forget himself as
far as possible.