A Treatise of Human Nature
A like reasoning will account for the idea of external existence. We may observe, that it
is universally allowed by philosophers, and is besides pretty obvious of itself, that
nothing is ever really present with the mind but its perceptions or impressions and ideas,
and that external objects become known to us only by those perceptions they occasion.
To hate, to love, to think, to feel, to see; all this is nothing but to perceive.
Now since nothing is ever present to the mind but perceptions, and since all ideas are
derived from something antecedently present to the mind; it follows, that it is impossible
for us so much as to conceive or form an idea of any thing specifically different. from
ideas and impressions. Let us fix our attention out of ourselves as much as possible: Let
us chase our imagination to the heavens, or to the utmost limits of the universe; we never
really advance a step beyond ourselves, nor can conceive any kind of existence, but those
perceptions, which have appeared in that narrow compass. This is the universe of the
imagination, nor have we any idea but what is there produced.
The farthest we can go towards a conception of external objects, when supposed
SPECIFICALLY different from our perceptions, is to form a relative idea of them,
without pretending to comprehend the related objects. Generally speaking we do not
suppose them specifically different; but only attribute to them different relations,
connections and durations. But of this more fully hereafter.[Part IV, Sect. 2.]