The conclusiveness of testimony is designated moral certainty. The value of testimony depends on three things. 1. On the nature of the subject. Some subjects are capable of more accurate observation than others. 2. On the powers and character of the observer--his ability to understand or note that of which he testifies--and his honesty in common matters. 3. On the number of our informers. Several persons are less likely to be imposed upon than one.
Testimony or moral certainty is inferior to physical certainty. A physical certainty bears uniformly the name of certainty, while a moral certainty is characterised as a probability. Great, very great may be the probability, still it is less in reliableness than a physical certainty. The evidence of Cato or Aristides would be very conclusive--yet somewhat less certain than that which our own senses have proved.
The conclusions from moral certainties are obtained like other conclusions, by induction. The induction from moral facts is like the induction from physical facts, with this difference--that the conclusions from moral facts are probabilities, like the facts on which they are founded. Whatever has physical certainty in its favour is considered demonstrable, and when sufficient probable evidence is adduced in favour of a proposition, it is considered to be fairly proved. Some persons, biased by the strictness of mathematical proof, insist upon the same accuracy in moral investigations. I have elsewhere pointed out the juvenility and infatuation of this error. Insist upon demonstration where the nature of the questions admits it. Less should not, in such case, suffice. Accept probability where probability is the sole evidence attainable. Never ask more than reason can grant. We must admit gradations of validity. What we are conscious of, we _know_. All we receive on testimony, we _believe_. Physical certainty is _knowledge_: moral certainty, _belief_. Hume remarks, in his 'Essay on Probabilities,' that 'Mr. Locke divides all arguments into demonstrative and probable. In this view, we must say, that it is only probable all men must die, or that the sun will rise to-morrow. But to conform our language more to common us, we ought to divide arguments into _demonstrations, proofs, and probabilities_. By proofs, meaning such arguments from experience as leave no room for doubt or opposition.'