Handbook of the History of Logic: - Fordham University Faculty

Medieval Modal Theories and Modal Logic 549
3 LATER MEDIAEVAL DEVELOPMENTS
3.1 Modalities in Philosophy and Theology
It has been assumed that the increasing interest in modal syllogistic and modal
logic in general in early fourteenth century was associated with certain philosophical
and theological developments which added to the interest in modal theories.
These often involved the idea of simultaneous alternatives which did not play a
significant role in mid-thirteenth-century essentialist theory.
Following the twelfth-century model, Henry of Ghent (d. 1293) applied obligations
logic to Trinitarian doctrine by assuming doctrinally impossible positions in
order to see what followed from them and what did not. 161 This was an influential
idea. Several early fourteenth-century authors found an obligational analysis as
a useful tool for analysing conceptual connections between theological concepts.
The most popular version of this logic was called positio. It deals with how an
increasing set of true and false propositions might remain coherent in a disputation
in which an opponent puts forward a contingent and false initial proposition and
a respondent accepts this and accepts or denies other propositions in a logically
consistent way. Irrelevant propositions are granted, denied or doubted according
to the best knowledge of the respondent. Relevant propositions should be treated
in a consistent way. These either follow from the initial position and/or what has
been granted and/or the opposites of what has been correctly denied or are incompatible
with them. Thirteenth-century positio rules denied that a now false but
possible position could refer to the time of an actual obligations discussion. This
was in agreement with the traditional doctrine of the necessity of the present. 162
As part of his new interpretation of modal concepts, John Duns Scotus dropped
this rule, a revision which made it possible to understand obligational answers as
partial descriptions of how things could be instead of regarding them as a internally
consistent set of propositions without a sensible interpretation. 163
Scotus’s revision of obligations rules was in agreement with his modal meta-
161 Henry of Ghent, Summae quaestionum ordinarium I.56 (Paris 1520, reprinted St. Bonaventure,
N.Y.: The Franciscan Institute; Louvain: Nauwelaerts; Padenborn: Schöningh, 1953),
vol. 2, 92r-v. One twelfth-century predecessor is Tractatus Emmeranus de impossibili positione,
edited by L.M. de Rijk in ‘Some Thirteenth Century Tracts on the Game of Obligation’, Vivarium
12 (1974), 94-123; an English translation by M. Yrjönsuuri in M. Yrjönsuuri (ed.), Medieval
Formal Logic, The New Synthese Historical Library 49 (Dordrecht: Kluwer, 2001), 217-23.
162 See note 104 above.
163 For Scotus’ revision, see Lectura I.39.1-5, 56, 59 in in Opera omnia, ed. C. Balić et al.
(Vatican Ciry: Typis Polyglottis Vaticanis, 1950-), vol. 17; an English translation in John Duns
Scotus, Contingency and Freedom: Lectura I.39, introduction, translation and commentary by
A. Vos Jaczn., H. Veldhuis, A.H. Looman-Graaskamp, E. Dekker and N.W. den Bok, The New
Synthese Historical Library 42 (Dordrecht: Kluwer Academic Publishers, 1994). For obligations
logic, see M. Yrjönsuuri, Obligationes: 14 th Century Logic of Disputational Duties, Acta
Philosophica Fennica 55 (Helsinki: Societas Philosophica Fennica, 1994) and note 83 above; for
obligations logic in fourteenth-century theology, see H. Gelber, It Could Have Been Otherwise.
Contingency and Necessity in Dominican Theology at Oxford 1300-350, Studien und Texte zur
Geistesgeschichte des Mittelalters 81 (Leiden: Brill, 2004).