f things in classes must greatly facilitate
recollection. But, besides this, a classification enables us easily to
run over all the contrasted and related things that we want to think of.
Explanation and classification both tend to rationalise the memory, and
to organise the mind in correspondence with Nature.

Every one knows how a poor mind is always repeating itself, going by
rote through the same train of words, ideas, actions; and that such a
mind is neither interesting nor practical. It is not practical, because
the circumstances of life are rarely exactly repeated, so that for a
present purpose it is rarely enough to remember only one former case; we
need several, that by comparing (perhaps automatically) their
resemblances and differences with the one before us, we may select a
course of action, or a principle, or a parallel, suited to our
immediate needs. Greater fertility and flexibility of thought seem
naturally to result from the practice of explanation and classification.
But it must be honestly added, that the result depends upon the spirit
in which such study is carried on; for if we are too fond of finality,
too eager to believe that we have already attained a greater precision
and comprehension than are in fact attainable, nothing can be more
petrific than 'science,' and our last state may be worse than the first.
Of this, students of Logic have often furnished examples.

Sec. 3. Classification may be either Deductive or Inductive; that is to
say, in the formation of classes, as in the proof of propositions, we
may, on the whole, proceed from the more to the less, or from the less
to the more general; not that these two processes are entirely
independent.

If we begin with some large class, such as 'Animal,' and subdivide it
deductively into Vertebrate and Invertebrate, yet the principle of
division (namely, central structure) has first been reached by a
comparison of examples and by generalisation; if, on the other hand,
beginning with individuals, we group them inductively into classes, and
these again into wider ones (as dogs, rats, horses, whales and monkeys
into mammalia) we are guided both in special cases by hypotheses as to
the best grounds of resemblance, and throughout by the general principle
of classification--to associate things that are alike and to separate
things that are unlike. This principle holds implicitly a place in
classification similar to that of causation in explanation; both