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INTRODUCTION.

PROBABLE EVIDENCE is essentially distinguished from demonstrative by this, that it admits of degrees; and of all variety of them, from the highest moral certainty, to the very lowest presumption. We cannot indeed say a thing is probably true upon one very slight presumption for it; because, as there may be probabilities on both sides of a question, there may be some against it: and though there be not, yet a slight presumption does not beget that degree of conviction, which is implied in saying a thing is probably true. But that the slightest possible presumption is of the nature of a probability, appears from hence; that such low presumption often repeated, will amount even to moral certainty. Thus a man's having observed the ebb and flow of the tide to-day, affords some sort of presumption, though the lowest imaginable, that it may happen again to-morrow : but the observation of this event for so many days, and months, and ages together, as it has been observed by mankind, gives us a full assurance that it will,

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

* Verisimile.

B

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, 'tis 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 observation reaches, come to pass at like distances of time, or place, or upon like occasions. Hence arises the belief, 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

The story is told by Mr. Locke in the Chapter of Probability.

be fluid and yielding: we, on the contrary, from analogy conclude, that there is no presumption at all against this: that 'tis 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 the 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 than even 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 reason

* See Chap. vi. Part II.