his assertion that there are three
departments, physics, biology, and psychology, each with its
characteristic questions, categories, and formulae. Of course, there are,
and equally, of course, physical laws will not cover biological facts;
nor will biological laws cover psychological ones. This is not due to
any occult cause, but to the simple fact that as each group of phenomena
has its characteristic features, each set of laws are framed to cover
the phenomena presented by that group. Otherwise there would be no need
of these special laws. It is astonishing how paralysing is the effect of
the theistic obsession on the minds of even scientific men, since it
leads them to ignore what is really a basic consideration in scientific
method.

Perhaps a word or two more on this topic is advisable. If it is
permissible to arrange natural phenomena in a serial order, we may place
them in succession as physical, chemical, biological, and psychological.
But these names represent no more than descriptions of certain features
that are to the group common, otherwise the grouping would be useless
and impossible. And it is part of the business of science to frame
"laws"--descriptions--of phenomena such as will enable us to express
their characteristic features in a brief formula. It is, therefore,
quite true to say that you cannot express vital phenomena in terms of
physics or chemistry. And no materialist who took the trouble to
understand materialism, instead of taking a statement of what it is from
an anti-materialist, ever thought otherwise. _Each specific group of
phenomena can only be covered by laws that belong to that group, and
which were framed for that express purpose._ A psychological fact can no
more be expressed in terms of chemistry than a physical fact can be
expressed in terms of biology. These truths are as plain to the
mechanist as they are to the vitalist. Mental life, the scientific
categories, are real to all; the only question at issue is that of their
origin.

To explain is to make intelligible, and in that sense all scientific
explanation consists in the establishing of equivalents. When we say
that A, B, C are the factors of D, we have asserted D is the equivalent
of A, B, C--plus, of course, all that results from the combination of
the factors. When we say that we have explained the formation of water
by showing it to be the product of H.2.O. we have shown that whether we
say "water" or use the chemical f