The much mentioned the modal fallacy is not a fallacy (that is, is a valid inference rule) if one accepts an exotic view about modalities and necessities that is logically implied by a particular understanding of infallible knowledge and a knower.
Some people seem to think that some known things are false and thus the need for a term like infallible knowledge, for that kind of knowledge that cannot be of false things. However that term “infallible knowledge” (and it’s under-term “infallible foreknowledge”) is subject to some interpretation. Is it best understood as:
A. If something is known, then it is necessarily true.
B. Necessarily, if something is known, then it is true.
Or equivalently, in terms of “cannot” instead of “necessarily”:
A. If something is known, then it cannot be false.
B. It cannot be false that, if something is known, then it is true.1
I contend that the second interpretation, (B), is the best. However suppose that one accepts the first, (A).
The assumption of the existence of a foreknower
Now let’s assume that there is someone that knows everything (which is the case), the knower. He posses infallible knowledge á la (A). Now we can work out the implications.
The foreknower exists and knows everything (that is the case):
1. There exists at least one person and that for all propositions, that a proposition is the case logically implies that that person knows that proposition.
Whatever is known is necessarily the case (A):
2. For all propositions and for all persons, that a person knows a proposition logically implies that that proposition is necessarily true.
Thus, every proposition that is the case is necessarily the case:
Thus, 3. For all propositions, that a proposition is the case logically implies that it is necessarily the case.
⊢ (∀P)(P⇒□P) [from 1, 2, HS]
Thus, everything that is logically possible is the case:
Thus, 4. For all propositions, that a proposition is logically possible logically implies that it is the case.
⊢ (∀P)(◊P⇒P) [from 3, others]2
Thus, everything that is logically possible is necessarily the case:
Thus, 5. For all propositions, that a proposition is logically possible logically implies that it is necessarily the case.
⊢ (∀P)(◊P⇒□P) [from 3, 4, HS]
This is called modal collapse. The acceptance of that all possibilities are necessarily the case.
Thus, the modal fallacy is no longer a fallacy:
Thus, 6. For all propositions (P) and for all propositions (Q), that a proposition (P) is the case, and that that proposition (P) logically implies a proposition (Q), logically implies that that proposition (Q) is necessarily the case.
⊢ (∀P)(∀Q)(P∧(P⇒Q)⇒□Q) [from 3]3
And so we can validly infer from a proposition being the case and that that proposition logically implies some other proposition to that that other proposition is necessarily the case.