The facility, with which each muscle changes from one associated tribe to
another, and that either backwards or forwards, is well observable in the
muscles of the arm in moving the windlass of an air-pump; and the slowness
of those muscular movements, that have not been associated by habit, may be
experienced by any one, who shall attempt to saw the air quick
perpendicularly with one hand, and horizontally with the other at the same
time.

3. In learning every kind of science we voluntarily associate many tribes
and trains of ideas, which afterwards are ready for all the purposes either
of volition, sensation, or irritation; and in some instances acquire
indissoluble habits of acting together, so as to affect our reasoning, and
influence our actions. Hence the necessity of a good education.

These associate ideas are gradually formed into habits of acting together
by frequent repetition, while they are yet separately obedient to the will;
as is evident from the difficulty we experience in gaining so exact an idea
of the front of St. Paul's church, as to be able to delineate it with
accuracy, or in recollecting a poem of a few pages.

And these ideas, thus associated into tribes, not only make up the parts of
the trains of volition, sensation, and irritation; but the same idea
composes a part of many different tribes and trains of ideas. So the simple
idea of whiteness composes a part of the complex idea of snow, milk, ivory;
and the complex idea of the letter A composes a part of the several
associated trains of ideas that make up the variety of words, in which this
letter enters.

The numerous trains of these associated ideas are divided by Mr. Hume into
three classes, which he has termed contiguity, causation, and resemblance.
Nor should we wonder to find them thus connected together, since it is the
business of our lives to dispose them into those three classes; and we
become valuable to ourselves and our friends, as we succeed in it. Those
who have combined an extensive class of ideas by the contiguity of time or
place, are men learned in the history of mankind, and of the sciences they
have cultivated. Those who have connected a great class of ideas of
resemblances, possess the source of the ornaments of poetry and oratory,
and of all rational analogy. While those who have connected great classes
of ideas of causation, are furnished with the powers of producing effects.
These are the men of active wisdom, who