what is a formula of calculation? It is presumably a statement that
something or other is true for natural occurrences. Take the simplest of
all formulae, Two and two make four. This--so far as it applies to
nature--asserts that if you take two natural entities, and then again
two other natural entities, the combined class contains four natural
entities. Such formulae which are true for any entities cannot result in
the production of the concepts of atoms. Then again there are formulae
which assert that there are entities in nature with such and such
special properties, say, for example, with the properties of the atoms
of hydrogen. Now if there are no such entities, I fail to see how any
statements about them can apply to nature. For example, the assertion
that there is green cheese in the moon cannot be a premiss in any
deduction of scientific importance, unless indeed the presence of green
cheese in the moon has been verified by experiment. The current answer
to these objections is that, though atoms are merely conceptual, yet
they are an interesting and picturesque way of saying something else
which is true of nature. But surely if it is something else that you
mean, for heaven's sake say it. Do away with this elaborate machinery of
a conceptual nature which consists of assertions about things which
don't exist in order to convey truths about things which do exist. I am
maintaining the obvious position that scientific laws, if they are true,
are statements about entities which we obtain knowledge of as being in
nature; and that, if the entities to which the statements refer are not
to be found in nature, the statements about them have no relevance to
any purely natural occurrence. Thus the molecules and electrons of
scientific theory are, so far as science has correctly formulated its
laws, each of them factors to be found in nature. The electrons are only
hypothetical in so far as we are not quite certain that the electron
theory is true. But their hypothetical character does not arise from the
essential nature of the theory in itself after its truth has been
granted.

Thus at the end of this somewhat complex discussion, we return to the
position which was affirmed at its beginning. The primary task of a
philosophy of natural science is to elucidate the concept of nature,
considered as one complex fact for knowledge, to exhibit the fundamental
entities and the fundamental relations between entities in terms of
whic