ifficult certainly, if not impossible, to speak of the
invisible operations of the mind or body, without expressing ourselves
in metaphor of some kind or other; and we are easily misled by
allusions to sensible objects, because when we comprehend the
allusion, we flatter ourselves that we understand the theory which it
is designed to illustrate. Whether we call ideas images in popular
language, or vibrations, according to Dr. Hartley's system, or modes
of sensation with Condillac, or motions of the sensorium, in the
language of Dr. Darwin, may seem a matter of indifference. But even
the choices of names is not a matter of indifference to those who wish
to argue accurately; when they are obliged to describe their feelings
or thoughts by metaphoric expressions, they will prefer the simplest;
those with which the fewest extraneous associations are connected.
Words which call up a variety of heterogeneous ideas to our minds, are
unfit for the purposes of sober reasoning; our attention is distracted
by them, and we cannot restrain it to the accurate comparison of
simple proportions. We yield to pleasing reverie, instead of exerting
painful voluntary attention. Hence it is probably useful in our
attempts to reason, especially upon metaphysical subjects, to change
from time to time our nomenclature,[64] and to substitute terms which
have no relation to our old associations, and which do not affect the
prejudices of our education. We are obliged to define with some degree
of accuracy the sense of new terms, and we are thus led to compare our
old notions with more severity. Our superstitious reverence for mere
symbols is also dissipated; symbols are apt to impose even upon those
who acknowledge their vanity, and who profess to consider them merely
as objects of vulgar worship.

When we call a class of our ideas _images_ and pictures, a tribe of
associations with painting comes into our mind, and we argue about
Imagination as if she were actually a paintress, who has colours at
her command, and who, upon some invisible canvass in the soul,
portrays the likeness of all earthly and celestial objects. When we
continue to pursue the same metaphor in speaking of the moral
influence of Imagination, we say that her _colouring_ deceives us,
that her _pictures_ are flattering and false, that she draws objects
out of proportion, &c. To what do all these metaphors lead? We make no
new discoveries by talking in this manner; we do not learn t