alteration: it was, therefore, nothing but its image, which was present
to the mind. These are the obvious dictates of reason; and no man, who
reflects, ever doubted, that the existences, which we consider, when we
say, _this house_ and _that tree_, are nothing but perceptions in the
mind, and fleeting copies or representations of other existences, which
remain uniform and independent.

119. So far, then, are we necessitated by reasoning to contradict or
depart from the primary instincts of nature, and to embrace a new system
with regard to the evidence of our senses. But here philosophy finds
herself extremely embarrassed, when she would justify this new system,
and obviate the cavils and objections of the sceptics. She can no longer
plead the infallible and irresistible instinct of nature: for that led
us to a quite different system, which is acknowledged fallible and even
erroneous. And to justify this pretended philosophical system, by a
chain of clear and convincing argument, or even any appearance of
argument, exceeds the power of all human capacity.

By what argument can it be proved, that the perceptions of the mind
must be caused by external objects, entirely different from them, though
resembling them (if that be possible) and could not arise either from
the energy of the mind itself, or from the suggestion of some invisible
and unknown spirit, or from some other cause still more unknown to us?
It is acknowledged, that, in fact, many of these perceptions arise not
from anything external, as in dreams, madness, and other diseases. And
nothing can be more inexplicable than the manner, in which body should
so operate upon mind as ever to convey an image of itself to a
substance, supposed of so different, and even contrary a nature.

It is a question of fact, whether the perceptions of the senses be