Appendix 13. (814).
Probability is frequently mentioned in the rules of conscience that I have given in this book. Moral science will be helped, therefore, if in addition to the schema of Certainty referred to in 510, I classify probability
I divide moral probability into seven classes:
First class: maximum probability. This kind of probability naturally produces a full opinionative assent whenever the opposite probability is so small as to be negligible, or, if not so small, escapes human attention, or, if observable, has no force to produce a significant fear of the contrary.
Second class: legitimate probability. This is required by those laws which allow a person to act when a condition prescribed by the law has been fulfilled. As we said (cf. 682, 684, 685), the probability varies according to the quality of the laws, and we should note that it is a species rather than a degree of probability.
Third class: probability greater than its opposite. This probability is seen by the intellect as greater than its contrary but lacks the power to give the spirit a full opinionative assent; it simply inclines the spirit. It is less than legitimate probability (which can make up for the lack of certainty), and provides only opinion.
Fourth class: equal probability on both sides. This is a case of perfect doubt, in which properly speaking we cannot have an opinion, because opposite opinions of equal probability incline the spirit to neither side. Opinion, according to its definition, is a 'consent given by the intellect to doubtful things for some apparent reason.'
Fifth class: lesser probability. This is the opposite of opinion, from which the spirit holds back. Here we should note the true sense of the moralists' words, 'more probable opinion, less probable opinion, equally probable opinion.' Opinion, relative to the person who has it, is always more probable than its opposite, otherwise he would not hold it. But when different opinions are present in different people, the person hearing them says one opinion is more probable than another, or less probable, or equally probable, according to the number of people who hold it and the weight of their authority. The person who classifies opinions in this way is not expressing his opinion, but judging the opinions of others; he believes he is making a judgment, not expressing an opinion.
Sixth class: probability opposed to legitimate probability. This is formed by the small degree of probability which legitimate probability lacks in order to be certainty.
Seventh class: least probability. This is that very small amount of probability which the greatest probability lacks in order to be certainty.
Fear is located in the spirit, not the mind. It can also come from the imaginary apprehension of a danger, although the mind clearly sees there is no reason for it. Consequently, fear has no place in a table of probability.