Handbook of the History of Logic: - Fordham University Faculty

Medieval Modal Theories and Modal Logic 517
until the moment of time to which they refer. Even the Stoics spoke about alternative
prospective possibilities which are not yet fixed at the level of known causes,
but they also regarded fate as an active potency which ultimately necessitates
everything. In the Peripatetic theory of diachronic modalities it is assumed that
there are transient individual alternative possibilities, but those which will not be
realized disappear. 27 When Boethius refers to chance, free choice, and possibility,
which restrict the domain of causal necessity, his examples include temporalized
modal notions which refer to diachronic prospective possibilities at a given moment
of time. 28 A temporally determinate prospective possibility may not be realized at
the time to which it refers, in which case it ceases to be a possibility. Boethius did
not develop a theory of simultaneous synchronic possibilities which remain intact
even when diachronic possibilities have vanished, insisting that only what is actual
at a certain time is possible at that time with respect to that time.
The model of diachronic modalities provided Boethius with a more satisfactory
tool for qualifying the necessity of the present than the frequency interpretation
did. Instead of arguing that when the condition of the conditionally necessary
proposition is removed, the proposition itself is contingent, he could also remark
that when the antecedent conditions of a temporally necessary state of affairs are
considered, it may be realized that it was not necessary before it was actualized. 29
(See also 2.3 below.)
1.4 Divine Modalities
While Boethius’s works formed the main source of knowledge about philosophical
modal theories for early medieval thinkers, Augustine’s doctrine of creation made
them aware of a theological discussion of possibility and necessity which was based
on different ideas. Augustine argued that God simultaneously created the first
things and the seminal reasons for later things out of nothing. The creation was
based on an eternal free act of God’s perfectly good will, and took place through
his omnipotence. In Augustine’s Trinitarian view, the Son is a perfect image of
the Father and, as the Word, the seat of the ideas of finite beings which in a
less perfect manner can imitate the highest being. The ideas refer to possible
actualization in the domain of mutability. In this sense the possibilities have an
ontological foundation in God’s essence. 30 This became the dominating conception
of theological modal metaphysics until Duns Scotus departed from it. 31
27 See R.W. Sharples, Alexander of Aphrodisias on Fate, text, translation and commentary
(London: Duckworth, 1983); S. Bobzien, Determinism and Freedom in Stoic Philosophy (Oxford:
Clarendon Press, 1998).
28 See, e.g., In Periherm. I, 106.11-14; 120.9-16; II, 190.14-191.2; 197.20-198.3; 203.2-11; 207.18-
25. 29In Periherm. II, 245.4-246.19. See also Thomas Aquinas, In Peri herm. I.15, 201; Albert
the Great, Comm. in Periherm. I.5.6, 421; Siger of Brabant, De necessitate et contingentia
causarum 32.4-18.
30 S. Knuuttila, ‘Time and Creation in Augustine’ in E. Stump and N. Kretzmann (eds.), The
Cambridge Companion to Augustine (Cambridge: Cambridge University Press, 2001), 103-115.
31 Knuuttila 1993, 135-6.