Fallacy of Four Terms
Brad Pitt is a star, and stars are giant balls of gas, so Brad Pitt is a giant ball of gas.
The fallacy of four terms looks like a valid argument form called “Barbara”:
A is B.
B is C.
Therefore, A is C.
For example:
Frogs are amphibians.
Amphibians are cold-blooded.
Therefore, frogs are cold-blooded. (valid)
This argument has three terms:
A: frogs
B: amphibians
C: cold-blooded
How many terms does the following argument have?
Brad Pitt is a star.
Stars are giant balls of gas.
Therefore, Brad Pitt is a giant ball of gas.
It looks like it has three, but it really has four:
A: Brad Pitt
B: star (celebrity)
C: star (celestial object)
D: giant ball of gas
Therefore, it commits the fallacy of four terms:
A is B.
C is D.
Therefore, A is D.
Here, the extra term sneaks in through equivocation (i.e., using the same word to mean two different things in the same argument).
Here’s an example of the fallacy of four terms without equivocation:
Frogs are amphibians.
Dogs are mammals.
Therefore, frogs are mammals.
This mistake is obvious. The fallacy of four terms is more likely to fool us when combined with equivocation.
Here's a more realistic example of the fallacy of four terms:
Gravity is a theory.
A theory is a hunch.
Therefore, gravity is a hunch.
While it looks like there are three terms, there are actually four:
A: gravity
B: theory (scientific meaning)
C: theory (colloquial meaning)
D: hunch
Back to the Formal Fallacy Handbook
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Courses

Fallacy Detectors
Develop the skills to tackle logical fallacies through a series of 10 science-fiction videos with activities. Recommended for ages 8 and up.

Social Media Simulator
Teach your kids to spot misinformation and manipulation in a safe and controlled environment before they face the real thing. Recommended for ages 9 and up.

A Statistical Odyssey
Learn about common mistakes in data analysis with an interactive space adventure. Recommended for ages 12 and up.

Logic for Teens
Learn how to make sense of complicated arguments with 14 video lessons and activities. Recommended for ages 13 and up.

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Learn to recognize, understand, and manage your emotions. Designed by child psychologist Ronald Crouch, Ph.D. Recommended for ages 5 to 8.
Worksheets

Logical Fallacies Worksheets and Lesson Plans
Teach your grades 3-7 students about ten common logical fallacies with these engaging and easy-to-use lesson plans and worksheets.

Symbolic Logic Worksheets
Worksheets covering the basics of symbolic logic for children ages 13 and up.

Elementary School Worksheets and Lesson Plans
These lesson plans and worksheets teach students in grades 2-5 about superstitions, different perspectives, facts and opinions, the false dilemma fallacy, and probability.

Middle School Worksheets and Lesson Plans
These lesson plans and worksheets teach students in grades 5-8 about false memories, confirmation bias, Occam’s razor, the strawman fallacy, and pareidolia.

High School Worksheets and Lesson Plans
These lesson plans and worksheets teach students in grades 8-12 about critical thinking, the appeal to nature fallacy, correlation versus causation, the placebo effect, and weasel words.

Statistical Shenanigans Worksheets and Lesson Plans
These lesson plans and worksheets teach students in grades 9 and up the statistical principles they need to analyze data rationally.