BOOK I PART IV
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 prin-
cipally 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 defini-
tion 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 con-
ceived may exist; and whatever is clearly con-