Elements of the philosophy of the human mind. By Dugald Stewart. Rev. and abridged, with critical and explanatory notes, for the use of colleges and schools. By Francis Bowen ...

INVENTION. 183
rely with confidence on his inventive powers whenever he is
called upon to exert them, he must have acquired, by previous
habits of study, a command over those classes of his ideas which
are subservient to the particular effort that he wishes to make.
In what manner this command is acquired, it is not possible,
perhaps, to explain completely; but it appears to me to be
chiefly in the two following ways. In the first place, by his
habits of speculation, he may have arranged his knowledge in
such a manner as may render it easy for him to combine, at
pleasure, all the various ideas in his mind which have any relation to the subject about which he is occupied: or, secondly,
he may have learned by experience certain general rules, by
means of which he can direct the train of his thoughts into
those channels, in which the ideas he is in quest of may be
most likely to occur to him.
I. The former of these observations I shall not stop to illustrate particularly at present, as the same subject will occur
afterwards under the article of memory. It is sufficient for my
purpose, in this chapter, to remark, that as habits of speculation
have a tendency to classify our ideas, by leading us to refer
particular facts and particular truths to general principles, and
as it is from an approximation and comparison of related ideas
that new discoveries in most instances result, the knowledge of
the philosopher, even supposing that it is not more extensive, is
arranged in a manner much more favorable to invention than
in a mind unaccustomed to system.
How much invention depends on a proper combination of the
materials of our knowledge, appears from the resources which
occur to men of the lowest degree of ingenuity, when they are
pressed by any alarming difficulty and danger, and from the
unexpected exertions, made by very ordinary characters, when
called to situations which rouse their latent powers. In such
cases, I take for granted, that necessity operates in producing
invention, chiefly by concenztrating the attention of the mind to
one set of ideas, by leading us to view these in every light, and
to combine them variously with each other. As the same idea
may be connected with an infinite variety of others by different