Handbook of the History of Logic: - Fordham University Faculty

Medieval Modal Theories and Modal Logic 523
since their truth-value is changeable. If God’s knowledge is described by using
tensed propositions, analogously to the articles of faith before and after the coming
of Christ, one should read them so that they signify the same. This became a wellknown
position, since it was also employed in Peter Lombard’s Sententiae. 54
The formulations by Peter Abelard, Gilbert of Poitiers, Peter Lombard and
Peter of Poitiers discussed above exemplify twelfth-century deviations from the
Aristotelian thesis ‘What is necessarily is when it is.’ This was traditionally understood
as implying the principle of the necessity of the present, which was not
questioned in ancient modal theories. Since God’s knowledge about contingent
things was regarded as unchangeable, the contingency of this knowledge also implied
the denial of the Aristotelian equation of immutability with necessity, a denial
regarded as an explicit doctrine of the nominales. 55
The idea of modal alternativeness was also discussed by Robert Grosseteste in
early thirteenth century. Grosseteste taught that the opposites of actualized contingent
things are no longer realizable possibilities, though they are possible in
the sense that they could have been included in God’s eternal providential choice.
Actual history is an explication of one of the divine alternatives with respect to
which things are primarily called necessary, possible or impossible. Modalities at
this basic level are called modalities ‘from eternity and without beginning’. Mathematical
truths are necessary in this way. In addition to these ‘simple’ necessities,
there are necessities and impossibilities which have a beginning and which are
eternal contingencies in the sense that God could have chosen their opposites. 56
The contingency of the divine acts of knowledge and will is based on an atemporal
causal priority between the power and its acts. 57
1.6 Future Contingents
Some Stoics took Aristotle to deny that future contingent propositions are true or
false, as Boethius reports in his second commentary on Aristotle’s De interpretatione
(208.1-4). Future contingent propositions were regarded as true or false in
Stoic logic, the Stoics taking the universally valid principle of bivalence to imply
the predetermination of all future events. (See Cicero, De fato, 20-1.) 58 Boethius
regarded the Stoic view of future contingent propositions as well as the Stoic
characterization of Aristotle’s position as false, his interpretation being based on
54 Sententiae I.12.193-223; I.13.192-220; cf. Peter Lombard, Sententiae.1.39.1; 41.3.
55 Ebbesen and Iwakuma 1992, 194
56 Robert Grosseteste, De libero arbitrio in Die philosophischen Werke des Robert Grosseteste,
ed. L. Baur, Beiträge zur Geschichte der Philosophie des Mittelalters 9 (Münster: Aschendorff,
1912), 168.26-170.33, 178.28-9; N. Lewis, ‘The First Recension of Robert Grosseteste’s De libero
arbitrio, Mediaeval Studies 53 (1991), 1-88.
57 Ibid. 178.24-6.
58 On the basis of a note in Simplicius’s commentary on Aristotle’s Categories (407.6-13), it
is argued that some Aristotelians also qualified the principle of bivalence; for this and other
ancient deviations from the principle, see R. Sorabji, Necessity, Cause, and Blame: Perspectives
on Aristotle’s Theory (Ithaca, NY: Cornell University Press, 1980), 92-3. For the Stoic views,
see also Bobzien 1998, 59-86.