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A Treatise of Human Nature

Since therefore all knowledge resolves itself into probability, and becomes at last of the
same nature with that evidence, which we employ in common life, we must now examine
this latter species of reasoning, and see on what foundation it stands.
In every judgment, which we can form concerning probability, as well as concerning
knowledge, we ought always to correct the first judgment, derived from the nature of the
object, by another judgment, derived from the nature of the understanding. It is certain a
man of solid sense and long experience ought to have, and usually has, a greater
assurance in his opinions, than one that is foolish and ignorant, and that our sentiments
have different degrees of authority, even with ourselves, in proportion to the degrees of
our reason and experience. In the man of the best sense and longest experience, this
authority is never entire; since even such-a-one must be conscious of many errors in the
past, and must still dread the like for the future. Here then arises a new species of
probability to correct and regulate the first, and fix its just standard and proportion. As
demonstration is subject to the controul of probability, so is probability liable to a new
correction by a reflex act of the mind, wherein the nature of our understanding, and our
reasoning from the first probability become our objects.
Having thus found in every probability, beside the original uncertainty inherent in the
subject, a new uncertainty derived from the weakness of that faculty, which judges, and
having adjusted these two together, we are obliged by our reason to add a new doubt
derived from the possibility of error in the estimation we make of the truth and fidelity of
our faculties. This is a doubt, which immediately occurs to us, and of which, if we would
closely pursue our reason, we cannot avoid giving a decision. But this decision, though it
should be favourable to our preceding judgment, being founded only on probability, must
weaken still further our first evidence, and must itself be weakened by a fourth doubt of
the same kind, and so on in infinitum: till at last there remain nothing of the original
probability, however great we may suppose it to have been, and however small the
diminution by every new uncertainty. No finite object can subsist under a decrease
repeated IN INFINITUM; and even the vastest quantity, which can enter into human
imagination, must in this manner be reduced to nothing. Let our first belief be never so
strong, it must infallibly perish by passing through so many new examinations, of which
each diminishes somewhat of its force and vigour. When I reflect on the natural fallibility
of my judgment, I have less confidence in my opinions, than when I only consider the
objects concerning which I reason; and when I proceed still farther, to turn the scrutiny
against every successive estimation I make of my faculties, all the rules of logic require a
continual diminution, and at last a total extinction of belief and evidence.
Should it here be asked me, whether I sincerely assent to this argument, which I seem to
take such pains to inculcate, and whether I be really one of those sceptics, who hold that
all is uncertain, and that our judgment is not in any thing possest of any measures of truth
and falshood; I should reply, that this question is entirely superfluous, and that neither I,
nor any other person was ever sincerely and constantly of that opinion. Nature, by an
absolute and uncontroulable necessity has determined us to judge as well as to breathe
and feel; nor can we any more forbear viewing certain objects in a stronger and fuller
light, upon account of their customary connexion with a present impression, than we can
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