is working-over process not only reviews and
strengthens past knowledge, keeping it from forgetfulness, but it
throws new light upon it and exposes it to a many-sided criticism. In
the first place familiar ideas should not be allowed to rest in the
mind _unused_. Like tools for service they must be kept bright and
sharp. One reason why so many of the valuable ideas we have acquired
have gradually disappeared from the mind is because they remained so
long unused that they faded out of sight. The old saying that
"repetition is the mother of studies" needs to be recalled and
emphasized. By being put in contact, with new ideas, old notions are
seen and appreciated in new relations. Facts that have long lain
unexplained in the mind, suddenly receive a _new interpretation_, a
vivid and rational meaning. Or the old meaning is intensified and
vivified by putting a new fact in conjunction with it.

Where the climate and products of the British Isles have been studied
in political geography, and later on, in physical geography, the gulf
stream is explained in its bearings on the climate of western Europe,
the whole subject of the climate of England is viewed from a new and
interesting standpoint. In arithmetic, where the sum of the squares of
the two sides of a right-angled triangle is illustrated by an example
and later on in geometry the same proposition is taken up in a
different way and proved as a universal theorem, new and interesting
light is thrown upon an old problem of arithmetic. In _United States
history_, after the Revolution has been studied, the biography of a man
like Samuel Adams throws much additional and vivid light upon the
events and actors in Boston and Massachusetts. The life of John Adams
would give a still different view of the same great events; just as a
city, as seen from different standpoints, presents different aspects.

3. We have thus far shown that new ideas are more easily understood and
assimilated when they are brought into close contact with what we
already know; and secondly, that our old knowledge is often explained
and illuminated by new facts brought to bear upon it. We may now
observe the result of this double action--_the welding_ of old and new
into one piece, the close mingling and association of all our
knowledge, _i.e._, its unity. Apperception, therefore, has the same
final tendency that was observed in the _inductive process_, the
unification of knowledge, the concentr