Dr Blacklock spoke of scepticism in morals and religion, with apparant uneasiness, as if he wished for more certainty. Dr Johnson, who had thought it all over, and whose vigorous understanding was fortified by much experience, thus encouraged the blind bard to apply to higher speculations what we willingly submit to in common life: in short, he gave him more familiarly the able and fair reasoning of Butler’s Analogy: 'Why, sir, the greatest concern we have in this world, the choice of our profession, must be determined without demonstrative reasoning. Human life is not yet so well known, as that we can have it. And take the case of a man who is ill. I call two physicians: they differ in opinion. I am not to lie down, and die between them: I must do something.'
James Boswell, Journal of a Tour to the Hebrides, entry for 17 August.]
The reference is to Butler's famous pronouncement in the Introduction to the Analogy:
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 to Life. (page iii)
Of course, when talking of 'probability' Butler is not thinking of probability in the sense of probability theory, but of verisimilitude.