ELEVENTH LECTURE
If there are future contingent events, the doctrine of Aristotle about the propositions which concern such events is the only correct one. These propositions are not true or false in a determinate sense; their truth remains indeterminate until the event takes place. Because truth is conformity of mind to what is. And an effect which is both contingent and future, that is to say, which is not infallibly contained in its cause, and which does not yet exist, has no being. Therefore, no assertion concerning it can be true or false in a determinate sense.
Generally speaking, the philosophers of antiquity, in these kinds of discussions, considered the relation between an effect and its cause, that is to say, a nature pre-ordered by itself to that effect.
In his polemics against stoics, Thomas Aquinas explained that a contingent event can be considered either as present and already determined, or as future and not yet determined. In the first case, it can be known with infallible certainty (Socrates sits down). In the second case, that is, in the state of contingency, it cannot be known with infallible certainty (Will Socrates sit down?) whatever the knowledge in question may be, even an absolutely perfect knowledge.
Taking again the remark of Aristotle in the sixth (fifth) Book of Metaphysics, Thomas Aquinas teaches that the stoic determinism (doctrine of fatality) would be true if these two principles were true: First, everything which arrives has its cause. Second, the cause being given, the effect follows in a necessary way. But these two principles are not true. First, what is causal as such has no cause, because it is rather not being than being; and second, a cause preordered to an effect can be hindered from giving this effect by the interference of another cause. If Stoics were right, we must assert: if somebody eats salted food, he will be thirsty; if he is thirsty, he will go out to drink; if he goes out to drink, he will meet thieves who will kill him; therefore it was necessary that this man be killed because he ate salted food. But these events are not bound together in a single series of causes each one necessitating by its very nature the following one. These events depend upon an irreducible plurality of causal lines, therefore the least event is not necessary, but contingent, and it cannot be foreknown with certainty in the cause.