The Masked Man Fallacy
Alias: Illicit Substitution of IdenticalsType: Formal Fallacy
| Forms | |
|---|---|
| a = b
Ca (where C is an intensional context). Therefore, Cb. |
Ca (where C is an intensional context).
Not-Cb. Therefore, it is not the case that a = b. |
| Examples | |
| The masked man is Mr. Hyde.
The witness believes that the masked man committed the crime. Therefore, the witness believes that Mr. Hyde committed the crime. |
The witness believes that the masked man committed the crime.
The witness doesn't believe that Mr. Hyde committed the crime. Therefore, Mr. Hyde is not the masked man. |
| Counter-Examples | |
| The masked man is Mr. Hyde.
The witness claims that the masked man committed the crime. Therefore, the witness claims that Mr. Hyde committed the crime. |
The witness claims that the masked man committed the crime.
The witness denies that Mr. Hyde committed the crime. Therefore, Mr. Hyde is not the masked man. |
Exposition:
Substitution of Identicals, also known as "Leibniz' Law", is a validating form of argument so long as the context in which it occurs is extensional, or referentially transparent. For instance, given that Mark Twain wrote Huck Finn and that Sam Clemens was the same person as Mark Twain, then Sam Clemens wrote Huck Finn. The context "x wrote Huck Finn" is extensional, which means that we can validly substitute identicals within it. In contrast, if Joe said "Mark Twain wrote Huck Finn", it does not follow that he said "Sam Clemens wrote Huck Finn", for he may have said no such thing.
A quoted context is an intensional—or, referentially opaque—context, as are such other contexts as:
- Propositional attitudes: belief, desire, fear, etc.
- Modal contexts: necessity, possibility, etc.
The Fallacy of Illicit Substitution of Identicals—or, more colorfully, "The Masked Man Fallacy"—is an application of Leibniz' Law within an intensional context. The most familiar uses of Substitution of Identicals are mathematical, where the contexts are always extensional. This may mislead one into thinking that substitution is valid in all contexts, but we have seen that this is not the case.
Source:
Ted Honderich (editor), The Oxford Companion to Philosophy, 1995.
