MONTY PYTHON AND

THE QUEST FOR THE PERFECT

FALLACY

The Witch’s Trial Argument

Activity adapted from Annenberg Classroom Fact Check

FactCheckEd.org

THE ARGUMENT

A L L W I T C H E S A R E T H I N G S T H A T C A N B U R N.

A L L T H I N G S T H A T C A N B U R N

A R E M A D E O F W O O D.

T H E R E F O R E , A L L W I T C H E S

A R E M A D E O F W O O D.

A L L T H I N G S T H A T A R E M A D E O F W O O D

A R E T H I N G S T H A T C A N F L OA T.

A L L T H I N G S T H A T W E I G H A S M U C H A S A

D U C K A R E T H I N G S T H A T C A N F L OA T.

S O , A L L T H I N G S T H A T W E I G H A S M U C H A S A

D U C K A R E T H I N G S T H A T A R E M A D E O F W O O D.

T H E R E F O R E , A L L W I T C H E S A R E T H I N G S

T H A T W E I G H A S M U C H A S A D U C K .

T H I S I S A T H I N G T H A T

W E I G H S A S M U C H A S A D U C K .

T H E R E F O R E , T H I S T H I N G I S A W I T C H .

FIRST ARGUMENT PART

All witches are things that can burn.

All things that can burn are made of wood.

Therefore, all witches are made of wood.

Is this a valid argument?

THIS IS A VALID ARGUMENT.

That is, (3) really does follow logically from

(1) and (2).

That’s not to say that it’s an especially

convincing argument because premise (2) is

rather obviously false.

Still, if (2) were true, then the conclusion

would have to be true as well.

So this step is valid but unsound.

1. All witches are things

that can burn.

2. All things that can

burn are made of wood.

3. Therefore, all witches

are made of wood.

SECOND ARGUMENT PART

All things that are made of wood are things that can float.

All things that weigh as much as a duck are things that can float.

So, All things that weigh as much as a duck are things that are

made of wood.

Is this a valid argument?

THIS IS A FALLACY

This argument commits the fallacy of the

undistributed middle.

The structure of the argument is the old

familiar

• All A is C.

• All B is C.

• Therefore, all A is B.

And that, of course, isn’t a valid argument.

A. All things that are made of

wood are things that can float.

B. All things that weigh as much as

a duck are things that can float.

C. So, All things that weigh as

much as a duck are things that

are made of wood.

THIRD ARGUMENT PART

Therefore, all witches are made of wood.

So all things that weigh as much as a duck are things that are made

of wood.

Therefore, all witches are things that weigh as much as a duck.

Is this a valid argument?

THIS IS A FALLACY

This argument has the same

problem as the second argument.

It’s also an undistributed middle.

• All A is C.

• All B is C.

• Therefore, all A is B.

All witches are made of wood.

So all things that weigh as much

as a duck are things that are made

of wood.

Therefore, all witches are things

that weigh as much as a duck.

FINAL ARGUMENT PART

All witches are things that weigh as much as a duck.

This thing is a thing that weighs as much as a duck.

Therefore, this thing is a witch.

Is this a valid argument?

THIS IS A FALLACY

The final argument:

Yes, once again, it’s an undistributed

middle .

All A is C.

All B is C.

Therefore, all A is B.

All witches are things that weigh

as much as a duck.

This thing is a thing that weighs

as much as a duck.

Therefore, this thing is a witch.