Fallacy of Modal Logic

Exposition:

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:

• Alethic Modalities: These include possibility and necessity, which were already mentioned, as well as impossibility and contingency. Some propositions are impossible, that is, necessarily false, whereas others are contingent, meaning that they are both possibly true and possibly false.
• Temporal Modalities: Historical and future truth or falsity. Some propositions were true/false in the past and others will be true/false in the future.
• Deontic Modalities: Obligation and permissibility. Some propositions ought to be true/false, while others are permissible.
• Epistemic Modalities: Knowledge and belief. Some propositions are known to be true/false, and others are believed to be true/false.

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.

Modal fallacies are formal fallacies in which modality plays a role in the fallaciousness of a type of argument.

Exposure:

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.

Subfallacy:

Modal Scope Fallacy

Resources:

• James Garson, "Modal Logic", Stanford Encyclopedia of Philosophy. A clear but technical survey of the field that assumes comfort with standard nonmodal logic.
• G. E. Hughes & M. J. Cresswell, A New Introduction to Modal Logic (Routledge, 1996). The standard introduction, which may be too much for novices.