o exhibit all sciences as one science, or--in case of
defeat--the disproof of such a possibility.

Again I will make a further simplification, and confine attention to the
natural sciences, that is, to the sciences whose subject-matter is
nature. By postulating a common subject-matter for this group of
sciences a unifying philosophy of natural science has been thereby
presupposed.

What do we mean by nature? We have to discuss the philosophy of natural
science. Natural science is the science of nature. But--What is nature?

Nature is that which we observe in perception through the senses. In
this sense-perception we are aware of something which is not thought and
which is self-contained for thought. This property of being
self-contained for thought lies at the base of natural science. It means
that nature can be thought of as a closed system whose mutual relations
do not require the expression of the fact that they are thought about.

Thus in a sense nature is independent of thought. By this statement no
metaphysical pronouncement is intended. What I mean is that we can think
about nature without thinking about thought. I shall say that then we
are thinking 'homogeneously' about nature.

Of course it is possible to think of nature in conjunction with thought
about the fact that nature is thought about. In such a case I shall say
that we are thinking 'heterogeneously' about nature. In fact during the
last few minutes we have been thinking heterogeneously about nature.
Natural science is exclusively concerned with homogeneous thoughts about
nature.

But sense-perception has in it an element which is not thought. It is a
difficult psychological question whether sense-perception involves
thought; and if it does involve thought, what is the kind of thought
which it necessarily involves. Note that it has been stated above that
sense-perception is an awareness of something which is not thought.
Namely, nature is not thought. But this is a different question, namely
that the fact of sense-perception has a factor which is not thought. I
call this factor 'sense-awareness.' Accordingly the doctrine that
natural science is exclusively concerned with homogeneous thoughts about
nature does not immediately carry with it the conclusion that natural
science is not concerned with sense-awareness.

However, I do assert this further statement; namely, that though natural
science is concerned with nature which is the terminus of
sen