Modal (Scope) Fallacy
A modal fallacy involves making a formal argument invalid by confusing the scope of what is actually necessary or possible.
THE IDEA
Description
Modal logic studies ways in which propositions can be true or false, the most common being necessity and possibility. Some propositions are necessarily true/false, and others are possibly true/false. A modal fallacy confuses the scope of necessity or possibility, changing what an argument establishes.
Examples
Example #1: Given that Debbie and TJ have two sons and two daughters, it follows that they have at least one son. The conditional inference is valid. The scope error would be to conclude that they necessarily have a son independently of that premise, in every possible circumstance.
Example #2: The original presidential-age argument is missing. A requirement attached to holding an office differs from a claim that a particular person necessarily has that age. Ordinary uses of 'must' may express the conditional requirement without committing a fallacy.
Exception
None.
When to use it
Evaluating arguments in political or economic debates when someone says 'we must do X' without clarifying whether it is a necessary logical truth or a contingent practical necessity.
Analyzing philosophical arguments, especially in metaphysics and philosophy of language, where modal logic is applied.
Checking your own reasoning for confusion between what is statistically likely and what is logically necessary.
Critiquing slippery slope arguments that imply a contingent outcome is an inevitable (necessary) conclusion.
FROM THEORY TO EVERYDAY LIFE
In practice
Debbie and TJ's Children
Given that Debbie and TJ have two sons and two daughters, they have at least one son. That conclusion follows from the premise. It would be a scope error to infer that they must have a son regardless of whether the premise holds.
Age of the President
The original argument is absent, so the scope of its necessity claim cannot be assessed. Distinguish an age requirement for office from a claim that a person necessarily has a particular age.
A MOMENT TO REFLECT
Put it to work.
When have you assumed that a specific outcome was inevitable, confusing a contingent fact for a necessary one?
How can you rephrase an argument to correctly distinguish between what is necessarily true and what is actually true in a specific case?
Compare 'Everyone who is President must be at least 35' with 'The President must be at least 35.' Could both express the same office requirement in context? What additional claim would create a scope error?
CONTINUE THE CONVERSATION
Source texts
- Logically Fallacious - The Ultimate Collection of Over 300 Logical Fallacies