h it always lies latent,
does not in ordinary circumstances arise in urgent form. It is therefore
evident that the fundamental conceptions of Science _do_ fall within the
purview of Philosophy.

The study of Physics _can_ be carried on practically as a study of
phenomena--of Heat, Colours, Sounds, Forces, etc., all of which are
kinds of phenomena--without the expression of any dogmatic and
formulated opinion as to their relation with Reality. Physics can speak
of mass and weight and avoid all reference to Matter; but there always
is, in scientific reasoning, an implicit reference to Reality, and it
facilitates, therefore, the expression of scientific reasoning, when the
account of a physical process is stated with reference to a supposed
reality, such as Matter. And in making such reference Science _is_
thinking of the thing-in-itself. It _is_ a reference beyond phenomena.

Heat, Light, Sound, Force, are names of classes of phenomena, and the
great discovery of Physics during the nineteenth century has been that
these are all transformable into each other, and bear definite numerical
relations to each other in proportion to which such transformations take
place. Science availing itself of this discovery, unifies its conception
of Nature and gives expression to the doctrine of the
inter-transmutability of the various classes of physical phenomena by
postulating an entity called Energy, and regarding the various classes
of phenomena as transmutations which this entity undergoes. But Science
has been reluctant to recognise that it is now entitled to dispense with
the postulation of Matter. The theory, as announced by the leading men
of science, has therefore been to the effect that there exist in the
physical universe _two_ real things--Matter and Energy--in place of one
only, as commonly supposed for so long.

Now we maintain, on the contrary, that such a statement of physical
theory is erroneous and redundant; that Science is not obliged to
postulate _two_ such entities; that the concept of Energy supplies all
her requirements; and that the employment of that conception obviates
the very serious contradictions which are involved in any assumption of
a real entity of the nature of Matter as ordinarily understood--a
conception of which the very description involves difficulties which
have perplexed thinking men for more than two centuries.

Our argument on this point involves consideration of the place occupied
by Ener