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 ...

86 CONCEPTION.
The speculative disbelief which we feel with respect to the
illusions of imagination, I conceive to be analogous to our speculative disbelief of the existence of the object exhibited to the
eye in this optical deception; as our belief that the illusions of
imagination are real, while that faculty occupied the mind exclusively, is analogous to the belief produced by the optical
deception while the attention is limited to our perception, and is
withdrawn from the circumstances in which the experiment is
made.
Orn the belief which attends perception. - These observations
lead me to take notice of a circumstance with respect to the belief accompanying perception, which it appears to me necessary
to state, in order to render Dr. Reid's doctrine on that subject
completely satisfactory. He has shown, that certain sensations
are, by a law of our nature, accompanied with an irresistible
belief of the existence of certain qualities of external objects.
But this law extends no further than to the present existence
of the quality; that is, to its existence while we feel the corresponding sensation. Whence is it then, that we ascribe to the
quality an existence independent of our perception? I apprehend we learn to do this by experience alone. WVe find that
we cannot, as in the case of imagination, dismiss or recall the
perception of an external object. If I open my eyes, I cannot
prevent myself from seeing the prospect which is before me. I
learn, therefore, to ascribe to the objects of my senses, not only
an existence at the time I perceive them, but an independent
and a permanent existence.
In sleep and madness, imagination is fully believed. - It is a
strong confirmation of this doctrine, that in sleep, when (as I
shall endeavor afterwards to show) the influence of the will
over the train of our thoughts is suspended, and when, of consequence, the time of their continuance in the mind is not regulated by us, we ascribe to the objects of imagination an independent and permanent existence, as we do when awake to the
objects of perception. The same thing happens in those kinds of
madness, in which a particular idea takes possession of the attention, and occupies it to the exclusion of every thing else. Indeed,