SECT。1 Of Scepticism with Regard to Reason
In all demonstrative sciences the rules are certain and infallible；but when we apply them，our fallible and uncertain faculties are very apt to depart from them，and fall into error。We must，therefore，in every reasoning form a new judgment，as a check or con-troul on our first judgment or belief；and must enlarge our view to comprehend a kind of history of all the instances，wherein our understanding has deceiv d us，compar d with those，wherein its testimony was just and true。Our reason must be considered as a kind of cause，of which truth is the natural effect；but such-a-one as by the irruption of other causes，and by the inconstancy of our mental powers，may frequently be prevented。By this means all knowledge degenerates into probability；and this probability is greater or less，according to our experience of the veracity or deceitfulness of our understanding，and according to the simplicity or intricacy of the question。
There is no Algebraist nor Mathematician so expert in his science，as to place en-tire confidence in any truth immediately upon his discovery of it，or regard it as any thing，but a mere probability。Every time he runs over his proofs，his confidence en-creases；but still more by the approbation of his friends；and is rais d to its utmost per-fection by the universal assent and applauses of the，learned world。Now tis evident，that this gradual encrease of assurance is nothing but the addition of new probabilities，and is deriv d from the constant union of causes and effects，according to past experience and observation。
In accompts of any length or importance，Merchants seldom trust to the，infallible certainty of numbers for their security；but by the artificial structure of the accompts，produce a probability beyond what is deriv d from the skill and experience of the ac-comptant。For that is plainly of itself some degree of probability；tho uncertain and vari-able，according to the degrees of his experience and length of the accompt。Now as nonewill maintain，that our assurance in a long numeration exceeds probability，I may safely affirm，that there scarce is any proposition concerning numbers，of which we can have a fuller security。For tis easily possible，by gradually diminishing the numbers，to reduce the longest series of addition to the most simple question，which can be form d，to an addition of two single numbers；and upon this supposition we shall find it impracticable to shew the precise limits of knowledge and of probability，or discover that particular number，at which the one ends and the other begins。But knowledge and probability are of such contrary and disagreeing natures，that they cannot well run insensibly into each other，and that because they will not divide，but must be either entirely present，or en-tirely absent。Besides，if any single addition were certain，every one woued be so，and consequently the whole or total sum；unless the whole can be different from all its parts。I had almost said，that this was certain；but I reflect that it must reduce itself，as well as every other reasoning，and from knowledge degenerate into probability。
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 ex-amine this latter species of reasoning，and see on what foundation it stands。
In every judgment，which we can form concerning probability，as well as concern-ing knowledge，we ought always to correct the first judgment，deriv d from the nature of the object，by another judgment，deriv d from the nature of the understanding。Tis cer-tain 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 au-thority 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 deriv d 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 deriv d from the possibility of error in the estimation we make of the truth and fidelity ofour faculties。
This is a doubt，which immediately occurs to us，and of which，if we woued close-ly pursue our reason，we cannot avoid giving a decision。But this decision，tho it shoued be favourable to our preceding judgment，being founded only on probability，must weaken still further our first evidence，and must itself be weaken d by a fourth doubt of the same kind，and so on in infinitum：till at last there remain nothing of the o-riginal 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 imagi-nation，must in this manner be reduc d to nothing。Let our first belief be never so strong，it must infallibly perish by passing thro so many new examinations，of which each diminishes somewhat of its force and vigour。When I reflect on the natural fallibili-ty 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。
Shou d it here be ask d 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 meas-ures of truth and falshood；I shou d 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 determin d 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 sur-rounding 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 endeavour d by arguments to establish a faculty，which nature has ante-cedently implanted in the mind，and render d unavoidable。
My intention then in displaying so carefully the arguments of that fantastic sect，is only to make the reader sensible of the truth of my hypothesis，that all our reasonings concerning causes and effects are deriv d from nothing but custom；and that belief is more properly an act of the，sensitive，than of the cogitative part of our natures。I havehere prov d，that the very same principles，which make us form a decision upon any subject，and correct that decision by the consideration of our genius and capacity，and of the situation of our mind，when we examin d that subject；I say，I have prov d，that these same principles，when carry d farther，and apply d to every new reflex judgment，must，by continually diminishing the original evidence，at last reduce it to nothing，and utterly subvert all belief and opinion。If belief，therefore，were a simple act of the thought，without any peculiar manner of conception，or the addition of a force and vi-vacity，it must infallibly destroy itself，and in every case terminate in a total suspense of judgment。But as experience will sufficiently convince any one，who thinks it worth while to try，that tho he can find no error in the foregoing arguments，yet he still contin-ues to believe，and think，and reason as usual，he may safely conclude，that his rea-soning and belief is some sensation or peculiar manner of conception，which tis impossi-ble for mere ideas and reflections to destroy。
SECT。2 Of Scepticism with Regard to the Senses
Thus the sceptic still continues to reason and believe，even tho he asserts，that he cannot defend his reason by reason；and by the same rule he must assent to the principle concerning the existence of body，tho he cannot pretend by any arguments of philosophy to maintain its veracity。Nature has not left this to his choice，and has doubtless，esteem d it an affair of too great importance to be trusted to our uncertain reasonings and speculations。We may well ask，What causes induce us to believe in the existence of body？but tis in vain to ask，Whether there be body or not？That is a point，which we must take for granted in all our reasonings。
The subject，then，of our present enquiry is concerning the causes which induce us to believe in the existence of body：And my reasonings on this head I shall begin with a distinction，which at first sight may seem superfluous，but which will contribute very much to the perfect understanding of what follows。We ought to examine apart those two questions，which are commonly confounded together，viz。Why we attribute a continu d existence to objects，even when they are not present to the senses；and why we suppose them to have an existence DISTINCT from the mind and perception。Under this last head I comprehend their situation as well as relations，their external position as well as the independence of their existence and operation。These two questions concerning the continu d and distinct existence of body are intimately connected together。For if the objects of our senses continue to exist，even when they are not perceiv d，their existence is of course independent of and distinct from the perception：and vice versa，if their ex-istence be independent of the perception and distinct from it，they must continue to ex-ist，even tho they be not perceiv d。But tho the decision of the one question decides the other；yet that we may the more easily discover the principles of human nature，from whence the decision arises，we shall carry along with us this distinction，and shall con-sider，whether it be the senses，reason，or the imagination，that produces the opinion of a continu d or of a distinct existence。These are the only questions，that are intelligible on the present subject。For as to the notion of external existence，when taken for some-thing specially different from our perceptions，we have already shewn its absurdity。
To begin with the SENSES，tis evident these faculties are incapable of giving rise to the notion of the continu d existence of their objects，after they no longer appear to the senses。For that is a contradiction in terms，and suppose that the senses continue to op-erate，even after they have ceas d all manner of operation。These faculties，therefore，if they have any influence in the present case，must produce the opinion of a distinct，not of a continu d existence；and in order to that，must present their impressions either as images and representations，or as these very distinct and external existences。