There is a species of scepticism, antecedent to all study and
philosophy, which is much inculcated by Des Cartes and others, as a
sovereign preservative against error and precipitate judgement. It
recommends an universal doubt, not only of all our former opinions and
principles, but also of our very faculties; of whose veracity, say
they, we must assure ourselves, by a chain of reasoning, deduced
from some original principle, which cannot possibly be fallacious or
deceitful. But neither is there any such original principle which
has a prerogative above others, that are self-evident and
convincing: or if there were, could we advance a step beyond it, but
by the use of those very faculties, of which we are supposed to be
already diffident. The Cartesian doubt, therefore, were it ever
possible to be attained by any human creature (as it plainly is not)
would be entirely incurable; and no reasoning could ever bring us to a
state of assurance and conviction upon any subject.

  It must, however, be confessed, that this species of scepticism,
when more moderate, may be understood in a very reasonable sense,
and is a necessary preparative to the study of philosophy, by
preserving a proper impartiality in our judgements, and weaning our
mind from all those prejudices, which we may have imbibed from
education or rash opinion. To begin with clear and self-evident
principles, to advance by timorous and sure steps, to review
frequently our conclusions, and examine accurately all their
consequences; though by these means we shall make both a slow and a
short progress in our systems; are the only methods, by which we can
ever hope to reach truth, and attain a proper stability and
certainty in our determinations.

  117. There is another species of scepticism, consequent to science
and enquiry, when men are supposed to have discovered, either the
absolute fallaciousness of their mental faculties, or their
unfitness to reach any fixed determination in all those curious
subjects of speculation, about which they are commonly employed.
Even our very senses are brought into dispute, by a certain species of
philosophers; and the maxims of common life are subjected to the
same doubt as the most profound principles or conclusions of
metaphysics and theology. As these paradoxical tenets (if they may
be called tenets) are to be met with in some philosophers, and the
refutation of them in several, they naturally excite our curiosity,
and make us enquire into the arguments, on which they may be founded.

  I need not insist upon the more trite topics, employed by the
sceptics in all ages, against the evidence of sense; such as those
which are derived from the imperfection and fallaciousness of our
organs, on numberless occasions; the crooked appearance of an oar in
water; the various aspects of objects, according to their different
distances; the double images which arise from the pressing one eye;
with many other appearances of a like nature. These sceptical
topics, indeed, are only sufficient to prove, that the senses alone
are not implicitly to be depended on; but that we must correct their
evidence by reason, and by considerations, derived from the nature
of the medium, the distance of the object, and the disposition of
the organ, in order to render them, within their sphere, the proper
criteria of truth and falsehood. There are other more profound
arguments against the senses, which admit not of so easy a solution.

  118. It seems evident, that men are carried, by a natural instinct
or prepossession, to repose faith in their senses; and that, without
any reasoning, or even almost before the use of reason, we always
suppose an external universe, which depends not on our perception, but
would exist, though we and every sensible creature were absent or
annihilated. Even the animal creation are governed by a like
opinion, and preserve this belief of external objects, in all their
thoughts, designs, and actions.

  It seems also evident, that, when men follow this blind and powerful
instinct of nature, they always suppose the very images, presented
by the senses, to be the external objects, and never entertain any
suspicion, that the one are nothing but representations of the
other. This very table which we see white, and which we feel hard,
is believed to exist, independent of our perception, and to be
something external to our mind, which perceives it. Our presence
bestows not being on it: our absence does not annihilate it. It
preserves its existence uniform and entire, independent of the
situation of intelligent beings, who perceive or contemplate it.

  But this universal and primary opinion of all men is soon
destroyed by the slightest philosophy, which teaches us, that
nothing can ever be present to the mind but an image or perception,
and that the senses are only the inlets, through which these images
are conveyed, without being able to produce any immediate
intercourse between the mind and the object. The table, which we
see, seems to diminish, as we remove farther from it: but the real
table, which exists independent of us, suffers no 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