o phenomena, and permits no inference concerning
the nature of the "Unknowable." The _Principles of Biology_ take up the
phenomena of life. Life is defined as the "continuous adjustment of
internal relations to external relations." No attempt is made to explain
its origin, yet (in the words of Mr. Sully) it is clear that the lowest
forms of life are regarded as continuous in their essential nature with
sub-vital processes. The evolution of living organisms, from the lowest to
the highest, with the development of all their parts and functions, results
from the co-operation of various factors, external and internal, whose
action is ultimately reducible to the universal law.

The field of _psychology_ is intimately allied with biology, and yet
istinguished from it. Mental life is a subdivision of life in general, and
may be subsumed under the general definition; but while biological truths
concern the connection between internal phenomena, with but tacit or
occasional recognition of the environment, psychology has to do neither
with the internal connection nor the external connection, but "the
connection between these two connections." Psychology in its subjective
aspect, again, is a field entirely _sui generis_. The substance of mind,
conceived as the underlying substratum of mental states, is unknowable; but
the character of those states of which mind, as we know it, is composed,
is a legitimate subject of inquiry. If this be carefully investigated, it
seems highly probable that the ultimate unit of consciousness is something
"of the same order as that which we call a nervous shock." Mind is
proximately composed of feelings and the relations between feelings;
from these, revived, associated, and integrated, the whole fabric of
consciousness is built up. There is, then, no sharp distinction between the
several phases of mind. If we trace its development objectively, in terms
of the correspondence between inner and outer phenomena, we find a gradual
progress from the less to the more complex, from the lower to the higher,
without a break. Reflex action, instinct, memory, reason, are simply
stages in the process. All is dependent on experience. Even the forms of
knowledge, which are _a priori_ to the individual, are the product
of experience in the race, integrated and transmitted by heredity, and
become organic in the nervous structure. In general the correspondence of
inner and outer in which mental life consists is mediate