The event will happen: Boethius has mistakenly inferred the necessity of this consequent ('it will happen'), when he is entitled only to infer the necessity of the whole conditional (‘if someone knows an event will happen, it will happen’).
Boethius the character is clearly taken in by this fallacious argument, and there is no good reason to think that Boethius the author ever became aware of the fallacy.
Intuitively, Boethius sees that the threat which divine prescience poses to the contingency of future events arises not just from the claim that God's beliefs about the future constitute knowledge, but also from the fact that they are beliefs about the future. There is a real problem here, because if God knows now what I shall do tomorrow, then it seems that either what I shall do is already determined, or else that I shall have the power tomorrow to convert God's knowledge today into a false belief.
Although his logical formulation does not capture this problem, the solution Boethius gives to Philosophy is clearly designed to tackle it.
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