George Campbell, answered from the texts and cited to the page.
Moral evidence is the species of rational evidence that governs all our conclusions about fact and about things without us — the whole domain, that is, of actual existence rather than necessary relation.1 The distinction I draw is this. Demonstrative evidence proceeds from the invariable properties and relations of general ideas; it is built on pure intellection and consists in an uninterrupted series of axioms.
Moral evidence, by contrast, is founded on the principles we derive from consciousness and common sense, improved by experience, and it proceeds on the general presumption that the course of nature in time to come will be similar to what it hath been hitherto.2 From that presumption it decides, in regard to particulars, concerning the future from the past, and concerning things unknown from things familiar to us.
This is why it is the proper province of rhetoric: wherever a speaker must establish a matter of fact, he is working in the domain of moral evidence. Three differences from demonstration deserve particular attention. The first is that moral truths, unlike demonstrated ones, are not necessary. That Caesar overcame Pompey, that the sun will rise tomorrow, that all men will die — the opposite of each is easily conceivable without any contradiction in terms.3
Demonstration admits no such latitude; what is demonstrated cannot be otherwise without absurdity. The second difference is that moral evidence admits of degrees, and demonstration does not. We ascend, by an insensible graduation, from bare possibility to probability, and thence to what I call the summit of moral certainty — and the conclusion of an argument may rest on any step of that ascent.4
Hence the result of demonstrative reasoning is denominated science; the result of moral reasoning is frequently entitled to no higher name than opinion, though it may on occasion carry a certainty that in practice is as immovable as any demonstration. That last point wants illustration, because it surprises. Suppose a man of known probity and philosophical learning tells you, with a serious countenance, that the sun which sets today will never rise again.
Would you doubt the fact? You might doubt his sanity, but not the proposition he denies — and this shows that some conclusions resting on moral evidence alone are held with a firmness that no single contrary testimony, however credentialed, can shake.5 The force of uniform experience, accumulated over ages and confirmed by the universal agreement of observers, produces a certainty that is moral in its kind yet practically indistinguishable from demonstration in its effect.
The third difference is that moral evidence almost always admits of contrariety. Two demonstrations cannot genuinely oppose one another, for to suppose contraries demonstrable is to suppose the same proposition both true and false.6 But contrary experiences, contrary presumptions, contrary testimonies are the ordinary condition of our reasonings about fact.
The probability upon the whole is in the proportion which the evidence on the preponderating side bears to its opposite; when we say the evidence lies on one side, we mean only the overplus on comparing both.7 Under moral evidence I distinguish three principal kinds: experience, analogy, and testimony. Experience in the strict sense yields moral certainty when it is full and uniform — that iron sinks in water admits no doubt — and yields probability only when the instances are divided.8
Analogy is, in truth, reasoning from a more indirect experience. Testimony is a distinct original principle, not derived from experience of human veracity but antecedent to it — a point on which I have elsewhere pressed Mr Hume with some force. The whole account matters for rhetoric because a speaker who does not understand what kind of evidence his conclusion requires will misapply his means.
Where demonstration is possible, perspicuity of diction and precision of arrangement are all that is wanted; the proper art of the orator has little purchase there.9 It is in the vast territory of moral evidence — of fact, of probability, of testimony weighed against experience — that eloquence finds its work.
The proper province of rhetoric is the second or moral evidence; for to the second belong all decisions concerning fact, and things without us.The Philosophy of Rhetoric
Moral evidence is founded on the principles we have from consciousness and common sense improved by experience; and as it proceeds on this general presumption or moral axiom, that the course of nature in time to come will be similar to what it hath been hitherto, it decides, in regard to particulars, concerning the future from the past, and concerning things unknown from things familiar to us.The Philosophy of Rhetoric
Whereas, to these truths, which we acquire by moral evidence, 'Caesar overcame Pompey;' 'The sun will rise to-morrow;' 'All men will die,' the opposite assertions, though untrue, are easily conceivable, without changing in the least the import of the words, and therefore do not imply a contradiction.The Philosophy of Rhetoric
In moral reasoning, we ascend from possibility, by an insensible graduation, to probability, and thence in the same manner, to the summit of moral certainty. On this summit, or on any of the steps leading to it, the conclusion of the argument may rest.The Philosophy of Rhetoric
A man of known probity and good sense, and an astronomer and philosopher, bids you look at the sun as it goes down, and tells you, with a serious countenance, that the sun which sets to-day will never rise again upon the earth. What would be the effect of this declaration? I believe it might, as to the soundness of the man's intellect, but not as to the truth of what he said.The Philosophy of Rhetoric
if one demonstration were ever capable of being refuted, it could be solely by another demonstration... But, to suppose that contraries are demonstrable, is to suppose that the same proposition is both true and false, which is a manifest contradiction.The Philosophy of Rhetoric
In this case, the probability, upon the whole, is in the proportion which the evidence on the side that preponderates bears to its opposite. We usually say, indeed, that the evidence lies on such a side of the question, and not on the reverse; but by this expression is only meant the overplus of evidence on comparing both sides.The Philosophy of Rhetoric
our experience is either uniform or various. In the one case, provided the facts on which it is founded be sufficiently numerous, the conclusion is said to be morally certain. In the other, the conclusion built on the greater number of instances is said to be probable.The Philosophy of Rhetoric
Here rhetoric, it must be acknowledged, hath little to do. Simplicity of diction and precision in arrangement, whence results perspicuity, are, as was observed already, all the requisites.The Philosophy of Rhetoric