It is the fallacy of placing a proposition in the wrong modal scope,[1] most commonly confusing the scope of what is necessarily true.
The modal fallacy occurs when there is a confusion of the distinction between the two.
A fallacy of necessity is an informal fallacy in the logic of a syllogism whereby a degree of unwarranted necessity is placed in the conclusion.
The condition a) appears to be a tautology and therefore true.
The condition b) is a statement of fact about John which makes him subject to a); that is, b) declares John a bachelor, and a) states that all bachelors are unmarried.
John, of course, is always free to stop being a bachelor, simply by getting married; if he does so, b) is no longer true and thus not subject to the tautology a).
In this case, c) has unwarranted necessity by assuming, incorrectly, that John cannot stop being a bachelor.
Formally speaking, this type of argument equivocates between the de dicto necessity of a) and the de re necessity of c).
The argument is only valid if both a) and c) are construed de re.
[2] Using the formal symbolism in modal logic, the de dicto expression
The conclusion is false, since, even though Mickey Mouse is over 35 years old, there is no logical necessity for him to be.
If instead of adding a stipulation of necessity, the argument just concluded that Mickey Mouse is 35 or older, it would be valid.
Norman Swartz gave the following example of how the modal fallacy can lead one to conclude that the future is already set, regardless of one's decisions; this is based on the "sea battle" example used by Aristotle to discuss the problem of future contingents in his On Interpretation:[3]Two admirals, A and B, are preparing their navies for a sea battle tomorrow.
But the 'laws' of the excluded middle (no third truth-value) and of non-contradiction (not both truth-values), mandate that one of the propositions, 'A wins' and 'B wins', is true (always has been and ever will be) and the other is false (always has been and ever will be).
Suppose 'A wins' is today true.
Then whatever A does (or fails to do) today will make no difference; similarly, whatever B does (or fails to do) today will make no difference: the outcome is already settled.
Then no matter what A does today (or fails to do), it will make no difference; similarly, no matter what B does (or fails to do), it will make no difference: the outcome is already settled.
Thus, if propositions bear their truth-values timelessly (or unchangingly and eternally), then planning, or as Aristotle put it 'taking care', is illusory in its efficacy.
The future will be what it will be, irrespective of our planning, intentions, etc.Suppose that the statement "A wins" is given by
are both true at the same time) The fallacy here occurs because one assumes that
Thus, one believes that, since one of both events is logically necessarily true, no action by either can change the outcome.
Swartz also argued that the argument from free will suffers from the modal fallacy.