Probability arises from an opposition of contrary chances or causes, by which the mind is not allow'd to fix on either side, but is incessantly tost from one to another, and at one moment is determin'd to consider an object as existent, and at another moment as the contrary. The imagination or understanding, call it which you please, fluctuates betwixt the opposite views; and tho' perhaps it may be oftner turn'd to the one side than the other, 'tis impossible for it, by reason fo the opposition of causes or chances, to rest on either. The pro and con of the question alternately prevail; and the mind, surveying he object in its opposite principles, finds such a contrariety as utterly destroys all certainty and establish'd opinion.
In probable inference, Hume thinks, the mind concludes (tentatively) to the side that has "a superior number of views or chances on one side" ( 188.8.131.52). In other words, the imagination wavers; and accepts the side it finds itself on most when it is wavering.