Fallacy of Modal LogicType: Formal Fallacy
Modal logic is that branch of logic which studies logical relations involving modalities. Modalities are ways, so to speak, in which propositions can be true or false. The most commonly studied modalities are necessity and possibility, which are modalities because some propositions are necessarily true/false and others are possibly true/false. Types of modality include:
Most modalities are propositional functions―that is, they are functions which when applied to a proposition produce a proposition―like negation, but unlike negation in that they are not truth-functional. That is, you cannot determine the truth-value of a modal proposition based solely upon the truth-value of the proposition it contains. For instance, from the fact that a certain proposition is true it does not follow that it is necessarily true, nor that it isn't. Some true propositions are necessary, but others are not.
Since modalities are frequent topics in philosophy―alethic modalities in metaphysics, epistemic ones in epistemology, and deontic ones in ethics―modal fallacies are quite frequent in philosophical and pseudo-philosophical argumentation. So, while students of philosophy should, of course, study logic and fallacies in general, they should pay particular attention to modal fallacies including the subfallacy below.