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 ...

ATTENTION. 67
to either of them at pleasure, without being much disturbed by
the other. If we attempt to listen to both, we can understand
neither. The fact seems to be, that when we attend constantly
to one of the speakers, the words spoken by the other make no
impression on the memory, in consequence of our not attending
to them; and affect us as little as if they had not been uttered.
This power, however, of the mind to attend to either speaker at
pleasure, supposes that it is, at one and the same time, conscious
of the sensations which both produce.
Another well-known fact may be of use in illustrating the
same distinction. A person who accidentally loses his sight,
never fails to improve gradually in the sensibility of his touch.
Now, there are only two ways of explaining this. The one is,
that, in consequence of the loss of the one sense, some change
takes place in the physical constitution of the body, so as to improve a different organ of perception. The other, that the
mind gradually acquires a power of attending to and remembering those slighter sensations of which it was formerly conscious,
but which, from our habits of inattention, made no impression
whatever on the memory. No one, surely, can hesitate for a
moment, in pronouncing which of these two suppositions is the
most philosophical.
Having treated, at considerable length, of those habits in
which both mind and body are concerned, I proceed to make a
few remarks on some phenomena which are purely intellectual;
and which, I think, are explicable on the same principles with
those which have been now under our review.
The influence of attention on memory illustrated by phenomena
and habits purely intellectual. - Every person who has studied
the elements of geometry, must have observed many cases in
which the truth of a theorem struck him the moment he heard the
enunciation. I do not allude to those theorems, the truth of
joint action is necessary to enable him to recover his balance. The muscular contractions that were originally isolated come, as it were, to cling
together, and take place under a single volition, as when both eyes turn to
the right or left, or all the fingers of the hand open or shut, by one impulse.]