CHAP. XV.

Of Probability.

Probability

1. As demonstration is the showing the agreement or disagreement of two ideas, by is the appearthe intervention of one or more proofs, ance of which have a constant, immutable, and visi- agreement ble connexion one with another; so proba- upon fallible proofs. bility is nothing but the appearance of such an agreement or disagreement, by the intervention of proofs, whose connexion is not constant and immutable, or at least is not perceived to be so, but is, or appears for the most part to be so, and is enough to induce the mind to judge the proposition to be true or false, rather than the contrary. For example: in the demonstration of it a man perceives the certain immutable connexion there is of equality between the three angles of a triangle, and those intermediate ones which are made use of to show their equality to two right ones; and so by an intuitive knowledge of the agreement or disagreement of the intermediate ideas in each step of the progress, the whole series is continued with an evidence, which clearly shows the agreement or disagreement of those three angles in equality to two right ones: and thus he has certain knowledge that it is so. But another man, who never took the pains to observe the demonstration, hearing a mathematician, a man of credit, affirm the three angles of a triangle to be equal to two right ones, assents to it, i. e. receives it for true. In which case the foundation of his assent is the probability of the thing, the proof being such as for the most part carries truth with it: the man, on whose testimony he receives it, not being wont to affirm any thing contrary to, or besides his knowledge, especially in matters of this kind. So that that which causes his assent to this proposition, that the three angles of a triangle are equal to two right ones, that which makes him take these ideas to agree, without knowing

them to do so, is the wonted veracity of the speaker in other cases, or his supposed veracity in this.

It is to sup

of know

ledge.

§ 2. Our knowledge, as has been shown, ply the want being very narrow, and we not happy enough to find certain truth in every thing which we have occasion to consider; most of the propositions we think, reason, discourse, nay act upon, are such, as we cannot have undoubted knowledge of their truth; yet some of them border so near upon certainty, that we make no doubt at all about them; but assent to them as firmly, and act, according to that assent, as resolutely, as if they were infallibly demonstrated, and that our knowledge of them was perfect and certain. But there being degrees herein from the very neighbourhood of certainty and demonstration, quite down to improbability and unlikeness, even to the confines of impossibility; and also degrees of assent from full assurance and confidence, quite down to conjecture, doubt, and distrust: I shall come now, (having, as I think, found out the bounds of human knowledge and certainty) in the next place, to consider the several degrees and grounds of probability, and assent or faith.

we know

Being that 3. Probability is likeliness to be true, which makes the very notation of the word signifying us presume such a proposition, for which there be arthings to be true before guments or proofs, to make it pass or be received for true. The entertainment the them to be so. mind gives this sort of propositions, is called belief, assent, or opinion, which is the admitting or receiving any proposition for true, upon arguments or proofs that are found to persuade us to receive it as true, without certain knowledge that it is so. And herein lies the difference between probability and certainty, faith and knowledge, that in all the parts of knowledge there is intuition; each immediate idea, each step has its visible and certain connexion; in belief, not So. That which makes me believe is something extraneous to the thing I believe; something not evidently joined on both sides to, and so not manifestly showing the agreement or disagreement of those ideas that are under consideration.

with our own

§ 4. Probability then, being to supply the defect of our knowledge, and to guide of probabiThe grounds us where that fails, is always conversant lity are two; about propositions, whereof we have no cer- conformity tainty, but only some inducements to receive them for true. The grounds of it are, in short, these two following. First, the conformity of any thing with our own knowledge, observation, and experience.

experience, or the testi

mony of

others expe

rience.

Secondly, the testimony of others, vouching their observation and experience. In the testimony of others, is to be considered, 1. The number. 2. The integrity. 3. The skill of the witnesses. 4. The design of the author, where it is a testimony out of a book cited. 5. The consistency of the parts, and circumstances of the relation. 6. Contrary testimonies.

§ 5. Probability wanting that intuitive In this all evidence, which infallibly determines the the arguunderstanding, and produces certain know- ments pro ledge, the mind, if it would proceed ration- and con ought to be ally, ought to examine all the grounds of examined beprobability) and see how they make more or fore we come less for or against any proposition, before to a judgit assents to, or dissents from it; and upon a due balancing the whole, reject, or receive it, with a more or less firm assent, proportionally to the preponderancy of the greater grounds of probability on one side or the other. For example:

ment.

If I myself see a man walk on the ice, it is past probability; it is knowledge; but if another tells me he saw a man in England, in the midst of a sharp winter, walk upon water hardened with cold; this has so great conformity with what is usually observed to happen, that I am disposed by the nature of the thing itself to assent to it, unless some manifest suspicion attend the relation of that matter of fact. But if the same thing be told to one born between the tropics, who never saw nor heard of any such thing before, there the whole probability relies on testimony: and as the relators are more in number, and of more credit, and have no interest to

speak contrary to the truth; so that matter of fact is like to find more or less belief. Though to a man whose experience has always been quite contrary, and who has never heard of any thing like it, the most untainted credit of a witness will scarce be able to find belief. As it happened to a Dutch ambassador, who entertaining the King of Siam with the particularities of Holland, which he was inquisitive after, amongst other things told him, that the water in his country would sometimes, in cold weather, be so hard, that men walked upon it, and that it would bear an elephant if he were there. To which the king replied, " Hitherto I have believed "the strange things you have told me, because I look "upon you as a sober fairman, but now I am sure you

66

lye."

They being capable of

great va

riety,

§ 6. Upon these grounds depends the probability of any proposition: and as the conformity of our knowledge, as the certainty of observations, as the frequency and constancy of experience, and the number and credibility of testimonies, do more or less agree or disagree with it, so is any proposition in itself more or less probable. There is another, I confess, which though by itself it be no true ground of probability, yet is often made use of for one, by which men most commonly regulate their assent, and upon which they pin their faith more than any thing else, and that is the opinion of others: though there cannot be a more dangerous thing to rely on, nor more likely to mislead one; since there is much more falsehood and errour among men, than truth and knowledge. And if the opinions and persuasions of others, whom we know and think well of, be a ground of assent, men have reason to be Heathens in Japan, Mahometans in Turkey, Papists in Spain, Protestants in England, and Lutherans in Sweden. But of this wrong ground of assent I shall have occasion to speak more at large in another place.

CHAP. XVI.

Of the Degrees of Assent.

1. THE grounds of probability we have Our assent laid down in the foregoing chapter; as they ought to be are the foundations on which our assent is regulated by the grounds built, so are they also the measure whereby of probabiliits several degrees are, or ought to be regu- ty. lated only we are to take notice, that whatever grounds of probability there may be, they yet operate no farther on the mind, which searches after truth, and endeavours to judge right, than they appear; at least in the first judgment or search that the mind makes. I confess, in the opinions men have, and firmly stick to, in the world, their assent is not always from an actual view of the reasons that at first prevailed with them: it being in many cases almost impossible, and in most very hard, even for those who have very admirable memories, to retain all the proofs, which upon a due examination made them embrace that side of the question. It suffices that they have once with care and fairness sifted the matter as far as they could; and that they have searched into all the particulars, that they could imagine to give any light to the question: and with the best of their skill cast up the account upon the whole evidence; and thus having once found on which side the probability appeared to them, after as full and exact an enquiry as they can make, they lay up the conclusion in their memories, as a truth they have discovered; and for the future they remain satisfied with the testimony of their memories, that this is the opinion, that by the proofs they have once seen of it deserves such a degree of their assent as they afford it.

These cannot

§ 2. This is all that the greatest part of men are capable of doing, in regulating always be their opinions and judgments; unless a man actually in will exact of them, either to retain distinctly view, and in their memories all the proofs concerning

then we must.