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.]

PART III.  OF KNOWLEDGE AND PROBABILITY.

SECT.  I. OF KNOWLEDGE.

There are seven [Part I. Sect. 5.] different kinds of philosophical relation, viz.  resemblance, identity, relations of time and place, proportion in quantity or number, degrees in any quality, contrariety and causation.  These relations may be divided into two classes; into such as depend entirely on the ideas, which we compare together, and such as may be changed without any change in the ideas.  It is from the idea of a triangle, that we discover the relation of equality, which its three angles bear to two right ones; and this relation is invariable, as long as our idea remains the same.  On the contrary, the relations of contiguity and distance betwixt two objects may be changed merely by an alteration of their place, without any change on the objects themselves or on their ideas; and the place depends on a hundred different accidents, which cannot be foreseen by the mind.  It is the same case with identity and causation.  Two objects, though perfectly resembling each other, and even appearing in the same place at different times, may be numerically different:  And as the power, by which one object produces another, is never discoverable merely from their idea, it is evident cause and effect are relations, of which we receive information from experience, and not from any abstract reasoning or reflection.  There is no single phaenomenon, even the most simple, which can be accounted for from the qualities of the objects, as they appear to us; or which we coued foresee without the help of our memory and experience.