Handbook of the History of Logic: - Fordham University Faculty

Medieval Modal Theories and Modal Logic 521
metaphysical contingency of the future. It is assumed that when something is, it
is possible that it is not at that very instant of time at which it is actual. 46
1.5 Modality as Alternativeness
Augustine’s doctrine of God’s eternal choice involved an intuitive idea of modality
as alternativeness which influenced early medieval theological discussions of divine
omnipotence and omniscience. Some authors regarded this as a special theological
matter which did not affect the use of traditional ideas in other disciplines, an
attitude supported by the general Aristotle reception in the thirteenth century,
but there were twelfth-century thinkers who realized the philosophical significance
of this new modal conception.
According to Abelard, the possibilities and necessities belonging to the individuals
of a species are determined by their shared nature. What nature demands
or allows and what is repugnant to it is the same for all members of a species,
and what is possible is seen in what has taken place in the representatives of the
species. 47 One could regard these essentialist possibilities as abstract Philonian
possibilities, many of which never become or even can become actual in an individual.
48 This is one aspect of Abelard’s modal views, but there are others. He
assumes that what is actual is temporally necessary at a certain point of time as
no longer avoidable, but he also argues that unrealized alternatives are possible
at the same time in the sense that they could have happened at that time. Some
of the alternatives of a singular being are real counterfactual alternatives. These
are unrealizable because of some previous changes in the conditions of the subject.
There are also merely imaginable alternatives, such as Socrates’ being a bishop,
which never had a real basis in things. 49 (See also 2.3 below.)
Gilbert of Poitiers stresses the idea that natural regularities which are called
natural necessities are not absolute, since they are chosen by God and can be
overridden by divine power. This had become a widespread theological view in
the twelfth century. Gilbert explained it in the light of the Augustinian view of
God’s acting by divine will, which chooses between alternative providential plans,
and divine omnipotence as an executive power. It has been sometimes thought
46Sententiae I.38.2.
47Dialectica 193.36-194.3; 385.1-8.
48Philo’s definitions of modal concepts are described in Boethius’s commentary on De interpretatione
(II, 234.10-21). One of his examples was that it is possible for a piece of wood at
the bottom of the sea to be burnt, in virtue of the fitness of the subject. See also Alexander
of Aphrodisias, In Aristotelis Analyticorum priorum librum I commentarium, ed. M. Wallies,
Commentaria in Aristotelem Graeca 2.1 (Berlin: Reimer, 1883), 184.6-12; John Philoponus, In
Aristotelis Analytica Priora commentaria, ed. M. Wallies, Commentaria in Aristotelem Graeca
13.2 (Berlin: Reimer, 1905), 169.19-20.
49See the discussion of Logica ‘Ingredientibus’, 272.39-273.19, and some other texts in C.J.
Martin, ‘An Amputee is Biped. The Role of the Categories in the Development of Abelard’s
Theory of Possibility’ in J. Biard and I. Catach-Rosier (eds.), La Tradition médiévale des Categories
(XIIe-XIVe siècles), Philosophes médiévaux 45 (Louvain-la-Neuve: Éditions de l’Institut
Supérieur de Philosophie, Louvain and Paris: Peeters, 2003), 225-42.