230 HERBERT SPENCER

reaches a certain, or rather uncertain, degree of differ-
entiation, its functioning becomes behaviour; its
activities are such that we cannot interpret them
without using psychical terms, such as awareness or
intelligence. This expression of fuller life is asso-
ciated with the increased development of the nervous
system, and we have no knowledge of any psychical
life apart from nervous metabolism. Yet we remain
quite unable to think of any way by which the meta-
bolism of nerve-cells gives rise to what we know in
ourselves as sensations or perceptions, ideas or feelings.
Therefore while we see no reason to doubt the con-
tinuity of the individual development, we recognise as
fact of experience that the merely sentient embryo
becomes a thoughtful child, whose behaviour cannot
be formulated in terms of our present biological or
our present mechanical categories.

And as it is with the individual development, so it
is with the evolution of organisms $ when they exhibit
a certain, or rather uncertain, degree of differentiation
they behave in a way which we cannot interpret with-
out using psychical terms. We know of very simple
forms whose whole behaviour seems to be summed up
in one reflex action, at least if there is more we cannot
detect it; we know of other unicellular animals whose
behaviour is such that we are forced to say that they
seem to pursue the method of trial and error; and
from that level we know of a long inclined plane lead-
ing up to very alert intelligence. Again we see no
reason to doubt the continuity of the process, though
we recognise that at a certain level of organisation
the biological categories of metabolism and the like
are no longer sufficient to formulate the facts. Howter, the one is no less than the other