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 ...

EXTEIRNAL PERCEPTION. 35
thie object to the organ.of sense, or soe me edium to intervene bed
tween the object and organ, by means of which the former may
communicate an impulse to the latter, appears from the common modes of expression on the subject, which are to be found
in all languages. In our own, for example, we frequently hear
the vulgar speak of light strifcing the eye; not in consequence
of any philosophical theory they have been taught, but of their
own crude and undirected speculations. Perhaps there are
few men among those who have attended at all to the history
of their own thoughts, who will not recollect the influence of
these ideas, at a period of life long prior to the date of their
philosophical studies. Nothing, indeed, can be conceived more
simple and natural than their origin. When an object is
placed in a certain situation with respect to a particular organ
of the body, a perception arises in the mind; when the object
is removed, the perception ceases. Hence we are led to apprehend some connection between the object and the perception; and
as we are accustomed to believe, that matter produces its effects
by impulse, we conclude that there must be some material medium intervening between the object and organ, by means of
which the impulse is communicated from the one to the other.
- That this is really the case, I do not mean to dispute. - I
think, however, it is evident, that the existence of such a medium does not in any case appear ct priori: and yet the natural
prejudices of men have given rise to an universal belief of it,
long before they were able to produce any good arguments in
support of their opinion.
Nor is it only to account for the connection between the object and the organ of sense, that philosophers have had recourse
to the theory of impulse. They have imagined that the imipression on the organ of sense is communicated to the mind, in a simr
ilar manner. As one body produces a change in the state of
another by impulse, so it has been supposed, that the external
object produces perception, (which is a change in the state of
the mind,) first, by some material impression made on the organ
of sense; and, secondly, by some material impression communicated from the organ to the mind along the nerves and braimn