That which chiefly constitutes Probability is expressed in the word Likely, i. e. like some truth or true event; like it, in itself, in its evidence, in some more or fewer of its circumstances. For when we determine a thing to be probably true, suppose that an event has or will come to pass, it is from the mind's remarking in it a likeness to some other event, which we have observed has come to pass. And this observation forms, in numberless daily instances, a presumption, opinion, or full conviction, that such event has or will come to pass ; according as the observation is, that the like event has sometimes, most commonly, or always, so far as our ob-servation reaches, come to pass at like distances of time, or place, or upon like occasions. Hence arises the be-lief, that a child, if it lives twenty years, will grow up to the stature and strength of a man; that food will contribute to the preservation of its life, and the want of it for such a number of days, be its certain destruction. So likewise the rule and measure of, our hopes and fears concerning the success of our pursuits; our expectations that others will act so and so in such circumstances; and our judgment that such actions proceed from such principles; all these rely upon our having observed the like to what we hope, fear, expect, judge; I say, upon our having observed, the like, either with respect to others or ourselves. And thus, whereas, the prince* who had always lived in a warm climate naturally concluded in the way of analogy, that there was no such thing as water's becoming hard, because he had always observed it to be fluid and yielding: we, on the contrary, from analogy conclude, that there is no presumption at all against this: that it is supposable there may be frost in England any given day in January next; probable that there will on some day of the month; and that there is a moral certainty, i. e. ground for an expectation without any doubt of it in some part or other of the winter.
Probable evidence, in its very nature, affords but an imperfect kind of information; and is to be considered as relative only to beings of limited capacities. For nothing which is the possible object of knowledge, whether past, present, or future, can be probable to an infinite Intelligence ; since it cannot but be discerned absolutely as it is in itself, certainly true, or certainly false. But to us, probability is the very guide of life.
From these things it follows, that in questions of difficulty, or such as are thought so, where more satisfactory evidence cannot be had, or is not seen; if the result of examination be, that there appears upon the whole,; any the lowest presumption on one side, and none on the other, or a greater presumption on one side, though in the lowest degree greater; this determines the question, even in matters of speculation; and in matters of practice, will lay us under an absolute and formal obligation, in point of prudence and of interest, to act upon that presumption or low probability, though it be so low, as to leave the mind in very great doubt which is the truth. For surely a man is as really bound in prudence to do what upon the whole appears, according to the best of his judgment, to be for his happiness, as what he certainly knows to be so. Nay further, in questions of great consequence, a reasonable man will think it concerns him to remark lower probabilities and presumptions than these; such as amount to no more than showing one side of a question to be as supposable and credible as the other: nay, such as but amount to much less even than this. For numberless instances might be mentioned respecting the common pursuits of life, where a man would be thought, in a literal sense, distracted, who would not act, and with great application too, not only upon an even chance, but upon much less, and where the probability or chance was greatly against his succeeding
It is not my design to inquire further into the nature, the foundation, and measure of probability; or whence it proceeds that likeness should beget that presumption; opinion, and full conviction, which the human mind is formed to receive from it, and which it does necessarily produce in every one; or to guard against the errors, to which reasoning from analogy is liable. This belongs to the subject of Logic; and is a part of that subject which has not yet been thoroughly considered. Indeed I shall not take upon me to say, how far the extent, compass, and force, of analogical reasoning, can be reduced to general heads and rules; and the whole be formed into a system. But through so little in this way has been attempted by those who have treated of our intellectual powers, and the exercise of them; this does not hinder but that we may be, as we, unquestionably are, assured, that analogy is of weight, in various degrees, towards determining our judgment and our practice. Nor does it in any wise cease to be of weight in those cases, because persons, either given to dispute, or who require things to be stated with greater exactness than our faculties appear to admit of in practical matters, may find other cases in which it is not easy to say, whether it be, or be not, of any weight; or instances of seeming analogies, which are really of none. It is enough to the present purpose to observe, that this general way of arguing is evidently natural, just, and conclusive. For there is no man can make a question but that the sun will rise tomorrow, and be seen, where it is seen at all, in the figure of a circle, and not in that of a square.
Hence, namely from analogical reasoning, Origin has with singular sagacity observed, that he who believes the Scripture to have proceeded from him who is the Author of Nature, may well expect to find the same sort of difficulties in it, as are found in the constitution of Nature. And in a like way of reflection it may be added, that he who denies the Scripture to have been from God upon account of these difficulties, may, for the very same reason, deny the world to have been formed by him., On the other hand, if there be an analogy or likeness between that system of things and dispensation of Providence, which Revelation informs us of, and that system of things and dispensation of Providence, which Experience together with Reason informs us of, i. e. the known course of Nature; this is a presumption, that they have both the same author and cause; at least so far as to answer objections against the former's being from God, drawn from any thing which is analogical or similar to what is in the latter, which is acknowledged to be from him; for an Author of Nature is here supposed.