As every idea is derived from a precedent impression, had we any idea of the substance of our minds, we must also have an impression of it; which is very difficult, if not impossible, to be conceived.

For how can an impression represent a substance, otherwise than by resembling it?
And how can an impression resemble a substance, since, according to this philosophy, it is not a substance, and has none of the peculiar qualities or characteristics of a substance?
But leaving the question of what may or may not be, for that other what actually is, I desire those philosophers, who pretend that we have an idea of the substance of our minds, to point out the impression that produces it, and tell distinctly after what manner that impression operates, and from what object it is derived.

Is it an impression of sensation or of reflection?
Is it pleasant, or painful, or indifferent?
I Does it attend us at all times, or does it only return at intervals?
If at intervals, at what times principally does it return, and by what causes is it produced?
If instead of answering these questions, any one should evade the difficulty, by saying, that the definition of a substance is something which may exist by itself; and that this definition ought to satisfy us: should this be said, I should observe, that this definition agrees to every thing, that can possibly be conceived; and never will serve to distinguish substance from accident, or the soul from its perceptions.
For thus I reason.

Whatever is clearly conceived may exist; and whatever is clearly conceived, after any manner, may exist after the same manner.
This is one principle, which has been already acknowledged.

Again, every thing, which is different, is distinguishable, and every thing which is distinguishable, is separable by the imagination.