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 hinder ourselves from thinking as long, as we are awake, or seeing the surrounding bodies, when we turn our eyes towards them in broad sunshine.  Whoever has taken the pains to refute the cavils of this total scepticism, has really disputed without an antagonist, and endeavoured by arguments to establish a faculty, which nature has antecedently implanted in the mind, and rendered unavoidable.