[N.B I'm not dealing with the question of how accurate or faithful Caj. was to the texts of either STA or Scotus. See p. 246. for Cajetan's text.]
Thomas argues that the source of contingency in the universe is the supereminent perfection of God; Scotus argues that it is the contingency of the divine choice. Scotus’s “most powerful argument” is:
(1) A moving cause, inasmuch as it is moved, moves with necessity if it is moved with necessity: but (2) every secondary cause is moving inasmuch as it is moved by the first – so if it is moved necessarily it moves with necessity. So if the first cause does not cause contingently, nothing in the universe happens with contingency.
A second cause can be moved by the first in two ways: by a motion preceding the proximate action (praevia propriae actioni) like when a stick moves a stone by the motion of the hand; or by intrinsically cooperating (coperativa intrinsece) with the proximate action. What is said in the major premise (1) according to the first sense is false in the second sense, which is the only sense in which it is true in the minor (2). It is not necessary that when someone wills something or when the sun illumines something that it cooperate with the first cause by a preceding motion; rather it suffices (and is required) that it intrinsically cooperate by its choice or its illumination, because every act of cooperation is according to the nature of the thing that cooperates.
The basis of the distinction is (apparently) whether the second cause is attaining to its proximate effect by its nature (in which case it is a second cause working coperativa intrinsece) or not. Sticks do not push stones by nature, but the sun does naturally illumine and the will choose.
Cajetan’s general argument against the Scotist position is that it would follow from it that “contingency would not be from God as directly intended (a propositio) by an agent” since:
Effects directly intended by an agent need to be chosen by him, as is clear from Metaphysics Book IX.. But if contingency follows from God’s mode of willing it is not chosen by God but a consequence of his mode of choosing.