Many of the topics that seemed to most excite medieval thinkers, such as the relation of Providence and Divine Omniscience to human freedom or demonstrations of the existence of God, involved modal notions. Often in the preliminaries of such discussions, one can find rich discussions of the nature of possibility and necessity.
One prevalent conception of modality was closely aligned to a qualified commitment to the so-called Principle of Plenitude - that is, that no real possibility remains unactualized. Medieval thinkers qualified their assumption of the Principle because it was felt that an unrestricted version of the Principle would constrain God’s power and will. Nonetheless, in the natural order of things, it was often assumed that the Principle held. If A is a real possibility, then there was, is, or will be some time at which A is true. On this conception of modality, modal notions were effectively reduced to extensional terms that were merely means of classifying what happens in the one and only world at different moments of time (Knuuttila 1981:169). A is possible if and only if there is some time at which A obtains. A is impossible if and only if A never obtains. A is necessary if and only if A always obtains.
While prevalent, especially in the thirteenth century, this “statistical” model of possibility and necessity was not the only model countenanced by medieval thinkers. Anselm, for example, developed a conception of possibility based upon inherent capacity; necessity properly signified external constraint (Serene 1981). There are some passages in Abelard, which suggest that he did not wholeheartedly embrace the statistical model (cf. Log. ingr. II, 272-274). Abelard’s followers, the so-called nominates, seem to have consciously rejected the statistical model.
Duns Scotus was deeply critical of the statistical model of modality, and in its place, he developed a ‘‘synchronic’’ model of modality. On the statistical model, it was hard to avoid the consequence that every present thing or state-of-affairs was necessary. This followed from the fact that actualizing a potentiality takes time. If a is actually F at t, then in order to make it so that a is not F, a’s potentiality to become not F must be actualized. But if a potentiality takes time to actualize, it follows that given that a is F at t, a cannot be not F at t. Scotus, on the other hand, argued that even though a is actually F at t, a could be not F at t.
Scotus' position was not without precedent. The twelfth-century philosopher Gilbert of Poitiers, for example, also thought that there were synchronic alternative possibilities (Knuuttila 1993:75-82). Yet, Scotus presented a distinctively sophisticated articulation and forceful defense of the doctrine of that the present could have been different than it in fact is. His synchronic picture of modality had a tremendous influence on the development of theories of modality in the fourteenth century and onward.
Scotus is sometimes credited with anticipating the notion of a possible world, either in the sense that Leibniz had or in the late-twentieth-century sense of the notion. While Scotus does not use the term ‘‘possible world'' or any equivalent, he does defend the idea that God considers all compossible combinations of things and contingently wills that one of these maximally consistent composites of things be made true. However, Scotus did not believe, as Leibniz did, that objects mirror their whole universe, nor did he have the contemporary notion of ‘‘truth in a world’’ (Normore 2003:155).
Scotus’ synchronic conception of modality should perhaps be described as the doctrine of ‘‘the contingency of what has not passed into the past’’ (Normore 2003:135), for like many medieval thinkers, Scotus accepted the claim that the past is necessary. This latter proposition, however, was not left unchallenged. In the twelfth century, Peter Damian famously defended the claim that God’s will is absolutely unconstrained. This meant that God could even change the past. But given that Damian started with a statistical understanding of modality, his position implied that God could violate the laws of logic. A popular solution to this dilemma was that God could not do impossible things; to be able to do the impossible would be a sign of impotency, not power. Peter Abelard, notoriously, argued that God could not do anything more or other than what He in fact does (Theologia ‘‘Scholarium’’ III, §§ 27-60:511-526).
More generally, medieval thinkers tried to strike the right balance between God’s power, God’s will, and human freedom. God must be the necessary, first cause of all creations, but the manner in which God’s creative activity is necessary cannot be such that it either compromises God’s very own freedom or human freedom.
World History
