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repeated — the molecular condition of the brain must be 
gradually adjusted accordingly. By the law of heredity the 
tendencies to these adjustments must pass over into the brains 
of the succeeding generation. By constant exercise these 
adjustments would be so fixed as invariably to recur when 
their appropriate conditions should require, attended by their 
accompanying psychical experiences, till at last, as the result 
of the accumulated force of these recurring and inherited 
experiences, it has become absolutely necessary to the intel- 
lectual activity of the human race as we find it to think under 
them as accepted categories of scientific knowledge. The 
physiological origin and character of this theory of the con- 
ditions of science are sufficiently obvious. Every element in 
it is purely physiological — the nervous activity as the counter- 
part of mental activity; tendencies often awakened and fixed 
in the brain by repetition ; heredity by physiological trans- 
mission, and unconscious and necessary revival under every 
possible occasion. We do not assert that the theory, when 
physiologically viewed, is altogether coherent. Even though 
we should allow its principal assumptions to pass unquestioned, 
we do not find that it explains why so few of these relations 
between complex feelings or complex relations should origin- 
ally present themselves so frequently as to thrust aside many 
others — why the relations of time and space or causation 
should gain any advantage by their frequency, were there not 
some original necessity that determined them to be frequently 
and even uniformly present to the discerning mind. But if 
any such necessity for their frequent occurrence be admitted, 
then it must have existed before the intermediate action of 
the physiological agencies that are introduced to explain the 
permanence and the universality of the categories that have 
thus become the intellectual outfit of the race. Then again, 
heredity, while it transmits with strength and certainty, also 
transmits with tendencies to variation ; and the environment 
which receives the transmitted legacy of the past also fixes it 
with some discernible change. But this is contrary to the 
theory which holds the categories to be axiomatic and per- 
manent. 
If, on the other hand, we suppose the theory to be true, the 
consequences must be fatal to the authority of science itself. 
We see not why, under the operation of the physiological 
agencies supposed, new categories must not come into exist- 
ence which may displace or perhaps contradict those already 
recognised — nor why any species of so-called relations may 
not come into being ; nor why, under the operation of the 
inevitable tendency to change, the entire structure of axiomatic 
