A like reflection on general rules keeps us from augmenting our belief upon every encrease of the force and vivacity of our ideas.  Where an opinion admits of no doubt, or opposite probability, we attribute to it a full conviction:  though the want of resemblance, or contiguity, may render its force inferior to that of other opinions.  It is thus the understanding corrects the appearances of the senses, and makes us imagine, that an object at twenty foot distance seems even to the eye as large as one of the same dimensions at ten.

We may observe the same effect of poetry in a lesser degree; only with this difference, that the least reflection dissipates the illusions of poetry, and Places the objects in their proper light.  It is however certain, that in the warmth of a poetical enthusiasm, a poet has a, counterfeit belief, and even a kind of vision of his objects:  And if there be any shadow of argument to support this belief, nothing contributes more to his full conviction than a blaze of poetical figures and images, which have their effect upon the poet himself, as well as upon his readers.

SECT.  XI.  OF THE PROBABILITY OF CHANCES.

But in order to bestow on this system its full force and evidence, we must carry our eye from it a moment to consider its consequences, and explain from the same principles some other species of reasoning, which are derived from the same origin.

Those philosophers, who have divided human reason into knowledge and probability, and have defined the first to be that evidence, which arises from the comparison of ideas, are obliged to comprehend all our arguments from causes or effects under the general term of probability.  But though every one be free to use his terms in what sense he pleases; and accordingly in the precedent part of this discourse, I have followed this method of expression; it is however certain, that in common discourse we readily affirm, that many arguments from causation exceed probability, and may be received as a superior kind of evidence.  One would appear ridiculous, who would say, that it is only probable the sun will rise to-morrow, or that all men must dye; though it is plain we have no further assurance of these facts, than what experience affords us.  For this reason, it would perhaps be more convenient, in order at once to preserve the common signification of words, and mark the several degrees of evidence, to distinguish human reason into three kinds, viz.  that from knowledge, from proofs, and from probabilities.  By knowledge, I mean the assurance arising from the comparison of ideas.  By proofs, those arguments, which are derived from the relation of cause and effect, and which are entirely free from doubt and uncertainty.  By probability, that evidence, which is still attended with uncertainty.  It is this last species of reasoning, I proceed to examine.