THE SCIENTIFIC POSITION 231

it is that the activity of the nervous system does ex-
press itself in such a way, that we must use a new
set of terms—psychical ones—to cover the facts of
behaviour, no one has at present any conception,
A living creature behaves in such a way that we
cannot interpret what it does in terms of the motions
of the organic corpuscles which compose it. We do
not know how to formulate in physical terms its
growth, its development, its power of effective re-
sponse, its co-ordination of activities. Therefore we
introduce a special series of biological concepts, with-
out denying that a greater unity of formulation may
some day be attained either by a further simplification
of the biological concepts or by some change in the
physical concepts, such as, indeed, seems coming
about at present.

But again, a living creature behaves in such a way
that our biological concepts are insufficient to formu-
late its behaviour. We do not know how to interpret
what it does without psychological concepts of thinking,
feeling, and willing. It is possible that here, too, a
greater unity of formulation may some day be attained
either by a further simplification of the psychological
concepts or by some change in the biological concepts.
But sufficient unto the day is the science thereof.lular animals whose