Handbook of the History of Logic: - Fordham University Faculty

514 Simo Knuuttila
are not actualized, their ends are said to exist potentially (In Periherm. II, 453.10-
455.19). Necessarily actual potencies leave no room for the potencies of their
contraries. There are no contrary potencies in these cases, Boethius says, because
they would remain unrealized forever and the constitution of nature cannot include
elements which are in vain (II, 236.11-18). It is implied here that all natural types
of potency must show their reality through actualization. The potencies of nonnecessary
features of being do not exclude contrary potencies. They are not always
and universally actualized, but as potency-types even these are taken to fulfil the
criterion of genuineness mentioned (II, 237.1-5). This is in agreement with the
‘statistical’ model of modality.
The theory of active and passive potencies was originally meant to explain how
and why a singular change takes place. 17 Possibilities as active and passive potencies
are the dynamic aspects of an actual change. This background to the potency
paradigm made it a cumbersome model for singular possibilities. While it allowed
Aristotle and his medieval followers to speak about unrealized possibilities in the
sense of partial possibilities, i.e., as the correlates of active or passive potencies,
full singular possibilities were actualized when they could be actualized. Natural
passive potencies could not be actualized without an active power and they
were necessarily actualized when an active power activated them and there was
no external hindrance (Met. IX.5, 1048a5-21).
According to Thomas Aquinas, the generic natural potentialities, divided into
passive propensities (potentia passiva) and related activating principles (potentia
activa), are necessary features of things, determined by the nature of the subjects
in which they occur. These necessary structures cannot be violated, and therefore
miraculous events, which take place against the common course of nature, must
occur through a special supernatural causation. Corresponding to the supernatural
active power, there is a passive potentia obedientiae by which creatures may receive
exceptional influences from the divine cause. 18
Boethius believed that the passive and active conditions of a great number of
events were embedded in the antecedent causes of these events, but in criticizing
the Stoic causal determinism he also taught that according to an Aristotelian view
there were indeterminate events based on free choice, chance and ad utrumlibet
contingency which were not determined by preceding causes. 19 As for the efficient
causes, he distinguished between necessary causes which always produced their
effects and lower causes which may be prevented because of an indeterminacy
factor in the causal nexus of nature. 20 Related frequential ideas became popular
17 For agent and patient in Aristotle’s natural philosophy in general, see S. Waterlow, Nature,
Change and Agency in Aristotle’s Physics (Oxford: Clarendon Press, 1982), 159-203.
18 Summa theologiae I.82.1; In Metaph. V.6, 832-4; IX.1, 1782; for obedient powers, see De
potentia, ed. P.M. Pession in Quaestiones disputatae, vol. II (Turin: Marietti 1965), 1.3, ad 1;
6.1, ad 18.
19 See N. Kretzmann, ‘“Nos ipsi principia sumus”: Boethius and the Basis of Contingency’ in
T. Rudavsky (ed.), Divine Omniscience in Medieval Philosophy, Synthese Historical Library 25
(Dordrecht: Reidel, 1985) 28-9; Knuuttila 1993, 47-51.
20 In Periherm. II.197.10-198.3; In Ciceronis Topica, Patrologia Latina 64, 1148, 1152, trans-