Modal

This is the error of treating modal conditionals as if the modality applies only to the consequent of the conditional. "The" modal fallacy is the most well known of the infinitely many errors involving modal concepts, concepts such as necessity, possibility and so forth. A conditional is an if-then proposition. The consequent is the then-part, and the antecedent is the if-part.

Example:

If a proposition is true, then it can not be false. But if a proposition can not be false, then it is not only true but necessarily true. Therefore, if a proposition is true, then it's necessarily true.

The acceptable interpretation of the first premise, requires the modality to apply to the entire conditional in the sense that it really means "It's not possible that if a proposition is true, then it's false." However, the entire inference works only if the first premise is miscontrued as saying "If a proposition is true, then it is necessary that it's not false." To see that the misconstrual is unacceptable, pick a proposition such as "It's raining in Detroit." Let's suppose it actually is raining in Detroit. So, the antecedent of the misconstrual is true, but the consequent isn't, because it says "It is necessary that 'it's raining in Detroit' is not false." This isn't necessary, is it?