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 ...

44 EXTERNAL PERCEPTION.
ulate these perceptions, are different in tile case of the different
senses, and form a very curious object of philosophical inquiry.
Those, in particular, which regulate the acquired perceptions of
sight, lead to some very interesting and important speculations;
and, I think, have never yet been explained in a manner completely satisfactory.
III. Of the. Origin of our Knowledge. - The philosophers who
endeavored to explain the operations of the human mind by the
theory of ideas, and who took for granted, that, in every exertion of thought, there exists in the mind some object distinct
from the thinking substance, were naturally led to inquire
whence these ideas derive their origin; in particular, whether
they are conveyed to the mind from without by means of the
senses, or form part of its original furniture?
With respect to this question, the opinions of the ancients
were various; but as the influence of these opinions on the prevailing systems of the present age is not very considerable, it
is not necessary, for any of the purposes I have in view in this
work, to consicler them particularly. The moderns, too, have
been much divided on the subject; some holding with Des
Cartes, that the mind is furnished with certain innate ideas;
others, with Mr. Locke, that all our ideas may be traced from
sensation and reflection; and many, (especially among the later
metaphysicians in France,) that they may be all traced from
sensation alone.
Of these theories, that of 31r. Locke deserves more particularly our attention; as it has served as a basis of most of the
metaphysical systems which have appeared since his time; and
as the difference between it and the theory which derives all
our ideas from sensation alone, is rather apparent than real.
T/he theory of Locke. - In order to convey a just notion of
Air. Locke's doctrine concerning the origin of our ideas, it is
necessary to remark, that he refers to sensation all the ideas
which we are supposed to receive by tho external senses; our
ideas, for example, of colors, of sounds, of hardness, of extension,
of motion; and, in short, of all the qualities and modes of matter: to reflection, the ideas of our own mental operations which