On Probability (300-1599)

"The dialectician is concerned only with proceeding from propositions which are as acceptable as possible. These are propositions which seem true to most people and especially to the wise." (Thomas Aquinas, "Posterior Analytics", cca. 1268)

"But, from some pre-existing causes future effects do not follow necessarily, but usually. For instance, in most cases (ut in pluribus) a perfect human being results from the insemination of a mother by a man’s semen; sometimes, however, monsters are generated, because of some obstruction which overcomes the operation of the natural capacity." (Thomas Aquinas,"Summa contra gentiles", cca. 1259-1265)

"It is not probable that, among the vast number of the faithful, there would not be many people who would readily supply the needs of those whom they hold in reverence because of the perfection of their virtue." (Thomas Aquinas, "Summa contra gentiles", cca. 1259-1265)

"And yet the fact that in so many it is not possible to have certitude without fear of error is no reason why we should reject the certitude which can probably be had [quae probabiliter haberi potest] through two or three witnesses […]" (Thomas Aquinas, "Summa theologiae", cca. 1265-1274)

"[...] propositions are called probable because they are more known to the wise or to the multitude." (Thomas Aquinas, "Commentary on the Posterior Analytics", cca. 1270)

"It is sufficient that you obtain a probable certainty, which means that in most cases (ut in pluribus) you are right and only in a few cases (ut in paucioribus) are you wrong." (Thomas Aquinas, "Summa theologiae" , cca. 1265-1274) 

"The dialectician is concerned only with proceeding from propositions which are as acceptable as possible. These are propositions which seem true to most people and especially to the wise., (Thomas Aquinas, "Posterior Analytics", cca. 1268)

"The method of demonstration is therefore generally feeble and ineffective with regard to facts of nature (I refer to corporeal and changeable things). But it quickly recovers its strength when applied to the field of mathematics. For whatever it concludes in regard to such things as numbers, proportions and figures is indubitably true, and cannot be otherwise. One who wishes to become a master of the science of demonstration should first obtain a good grasp of probabilities. Whereas the principles of demonstrative logic are necessary; those of dialectic are probable." (John of Salisbury, "Metalogicon", 1159)

"Something is readily believable (probabilis) if it seems true to everyone or to the most people or to the wise – and of the wise, either to all of them or most of them or to the most famous and distinguished – or to an expert in his own field, for example, to a doctor in the field of medicine or to a pilot in the navigation of ships, or, finally, if it seems true to the person with whom one is having the conversation or who is judging it." (Boethius, De topicis, 1180)