# Four-term fallacy

The **four-term fallacy** is a logical fallacy that occurs when an argument that was intended to be a syllogism improperly uses four terms instead of three.

A valid syllogism contains three terms (here A, B, and C):

- Every A is a B.
- Every B is a C.
- Therefore every A is a C.

If a single instance of any of these terms is replaced by a fourth term, X, an invalid form results. For example:

- Every A is a B.
- Every B is a C.
- Therefore every A is an X.

Or:

- Every A is a B.
- Every X is a C.
- Therefore every A is a C.

And so forth.

The introduction of the extraneous term makes the argument fallacious. When done as transparently as in the examples above, it is generally easy to see that the argument does not hold. When done using some form of equivocation, the fallacy can be harder to spot.

## Example

Consider the following argument:

The proponent of this argument is attempting to discredit the theory of evolution by conflating the everyday use of *theory* as a mere hypothesis with the scientific use of the word as a well-established, thoroughly tested explanation. In this case the extraneous fourth term is the second use of the word *theory*.