On Opinion and Probability in Thomas Aquinas

With Michele Savarese.

Thomas Aquinas sees opinion (opinio), in its most technical sense, as a cognitive habit dealing with contingent matter (or with necessary matter known only contingently). But there is an evolution in Aquinas' view on this habit from his early works to the mature ones. Certainly, there are various constant elements, e.g., that it is acquired through dialectical reasoning. Or that, because of the contingency of its object, opinio is characterized by the lack of full certainty; this point is often famously expressed by saying it is an acceptance of one of two contradictory propositions cum formidine alterius (with the fear that the other holds). Nonetheless, as a careful exam of the most relevant passages of Scriptum, De Veritate, and Super Boetium de Trinitate on the one hand, and of Sententia libri Ethicorum, Secunda Secundae, and Expositio libri Posteriorum Analyticorum, on the other one, there is also a significant change. Indeed, in the former, the will does not play any role in opinio, and there is not a proper assent of the intellect to its object; therefore, opinio remains quite close to doubt (dubitatio). Whereas, in the latter, opinio is characterized by the assent of the intellect to its object, assent that is ordered by the will. Furthermore, there appears a cognitive habit between opinio and doubt, suspicio, which was absent in the early comparisons between fides, scientia, opinio, and dubitatio. So, it seems that, whereas in Aquinas' early works opinio was closer to doubt than to certainty and full knowledge, in the mature ones suspicio takes this role, and opinio gains a greater epistemic value. This higher status of opinio is also confirmed by other passages from Aquinas's mature works, especially the Secunda Secundae, again, and the Tertia Pars. Opinio is also intertwined with Aquinas' notion of probability. Indeed, the adjective probabilis, in its most technical meaning, characterizes the dialectic arguments producing opinion. Aquinas defines it only in terms of what is held by most people or by the wise, or in terms of what seems to be true always or in most cases. Clearly, these definitions are not suitable to grade contrasting opinions. So, in particular, they are quite far from the notions of probability used either in the debates on probabilism in moral theology or in Bayesian epistemology. But Aquinas himself recurs to the notion of probability also when dealing with opinion in the broader sense of the doctrine of some thinkers. In this context, he often (although more frequently in his early works) makes explicit use of grades of probability in a way that seems to somehow anticipate these much later approaches.