The Course of Reason

I Expose Penn Jillette’s Logic Trick

May 14, 2013

If you watch the first minute of this video clip you’ll see Penn Jillette perform a logic trick. I greatly respect his ability to point out the tricks magicians use to fool people. As a logician, I’d like to follow his lead and point out the trick he has used here. He claims that if you’re an agnostic, then you (pretty much) don’t believe in God. That is, he claims agnosticism implies atheism.

He is quite right to point out that agnosticism and atheism address completely separate issues. He is also correct when he says that agnosticism answers the question “Is there a God?” with the response, “I don’t know”.

He is also correct that atheism answers the question, “Do you believe in God?” in the negative.

Here’s his mistake. He claims ‘if you don’t know God exists, then you don’t believe God exists.’ This is clearly false. It states that knowledge is a necessary condition for belief. By contraposition (“if not p then not q” is the same as “if q then p”), we get the claim, “if you believe God exists, then you know God exists.” What self respecting atheist would endorse that claim? None! None I tell you! That would be bullshit!

 Penn Jillette in his Wizard attire

[Penn and Teller magic powers happen…]

Wait a minute. Maybe this does make sense.

Let’s back up and go through it slowly.

We’ve got two questions.

1. Does God exist?
Answer: I don’t know = Agnosticism. Check.
2. Do I believe God exists?
Answer: No = Atheism. Check.

But what if I give a different answer to question 1?

1. Does God exist?
Answer: Yes = Theism.

Sure, but is that all? In responding so positively, and so confidently, it seems like you’re really saying “yes God exists, and I know it be true.” So, in answering ‘yes’, it seems like the position of gnostic theism falls out. You can address both issues at once, depending on your answer.

What about this response:
1. Does God exist?
Answer: probably not.

This is the most common response you’ll get from atheists, because most atheists don’t want to answer “no” with such confidence, such certainty, because it feels like they’re saying “I know God exists,” and they don’t want to say that. So they say “probably not,” indicating that they don’t believe God exists without claiming knowledge of such.

So really, I think you can answer that first question in a way that reveals both your position on gnostic vs. agnostic and theist vs. atheist, and those various responses indicate that there is indeed a difference between the two issues. So can we get an answer indicating agnostic theism?

Can anyone ever reasonably answer the first question like this:
1. Does God exist?
Answer: probably… but I don’t know for sure.

This answer represents an agnostic theist. Penn Jillette doesn’t think you can be an agnostic theist, though. I think you can be an agnostic theist. What do you think Teller? 

Teller murdering Penn.

Clearly Teller agrees. How to explain this to Penn? What we need is a Good Analogy. Penn knows a lot about cards, so let’s use a card analogy.

You go to MIT and you are part of the MIT Blackjack Team. You are freaking amazing at counting cards. You’re a regular Rain Man. So you’re sitting there counting, and that count is really high, like +13, which means some face cards are about to start dropping. Pause.

1. Will one of the next few cards be a face card?
Answer: I don’t know = face card agnostic.

So, that means you don’t believe one of the next few cards will be a face card, right? Because face card agnosticism implies afacecardism. If so, you wouldn’t last long on the MIT Blackjack Team. They’d kick you off. For being a loser. When the count is +13, you signal for one of your teammates to come and start betting. The probability is high that one of the next few cards is a face card.

MIT blackjack team member, played by Kate Bosworth.

Not a loser.

But consider:

1. Will one of the next few cards be a face card?
Answer: Very probably, but I don’t know for sure.

Well, that seems like a perfectly fine answer.

What about:

2. Do you believe one of the next few cards will be a face card?
Answer: Yes.

Spacey, coach of the blackjack team.

Doesn’t play with losers.

So, there’s nothing about answering the first question with “I don’t know” that forces you to answer the second with “no”. And there is a perfectly logical reason for that.

Epistemology is the philosophical study of knowledge, and it is also one of the coolest of all philosophy subjects. The consensus is, and pretty much has been since before Socrates, that knowledge entails belief, but that belief does not entail knowledge.

This means that in order to know something, you’ve gotta at least believe it, but that believing is not enough to count as knowledge. If believing were enough to count as knowledge, then you’d have ridiculous things like, “If you believe God exists, then you know God exists.” Ridiculous, right?

Some of you are probably gearing up for the ACT or SAT, or maybe oven some other test involving a combination of capital letters. On those tests, you’ll have to do analogies. So, let’s do another analogy.

Belief is to knowledge as __________ is to ____________ .

a. shape ... triangle
b. mode of transportation .... car
c. death ... murder
d. injury ... assault

Of course, all of those are correct answers. If someone knows something, then they believe it. If someone has a car, then they have a mode of transportation. If someone is murdered, then they have died. If someone has been assaulted, then they have been injured.

These are all false, with (counterexamples in parentheses):
If someone doesn’t know something, then they don’t believe it (It might actually be false, but they wrongly believe it, in which case they don’t know it, but they believe it). If someone doesn’t have a car, then they don’t have a mode of transportation (bicycle). If someone has not been murdered, then they have not died (old age). If someone has not been assaulted, then they have not been injured (tripped on a tree root and broke an arm).

So, in conclusion, Penn Jillette’s logic trick rests on a claim that no self-respecting atheist would ever assert (if you believe X, then you know X), and it fails to recognize the proper relationship between knowledge and belief, namely that knowledge is merely a type of belief, just like murder is merely a type of death.

American Psycho demotivational.

Still, it’s a special kind of death, so make it special.

If you take a belief, add truth, justification, and some special sauce that avoids the Gettier problem, then you get knowledge. If you take a (human’s) death, add unlawfulness and another person as the cause, then you get murder. It’s that simple. Penn Jillette’s claim is formally identical to the claim, “If he died, then he was murdered.” This would make prosecutors’ jobs a lot easier, but thankfully everyone recognizes that it is false. It’s bullshit.

Penn and Teller: Bullshit!



About the Author: Seth Kurtenbach

Seth Kurtenbach's photo
Seth Kurtenbach is pursuing his PhD in computer science at the University of Missouri. His current research focuses on the application of formal logic to questions about knowledge and rationality. He has his Master's degree in philosophy from the University of Missouri, and is growing an epic beard in order to maintain his philosophical powers. You can email Seth at or follow him on Twitter: @SJKur.


#1 Somite on Thursday May 16, 2013 at 6:54am

Agnosticism on the question of god is really not a valid position because we know with absolute certainty that there is no evidence for any god.

If your answer is "I don't know" ask yourself where does the uncertainty comes from. Is there any evidence that supports the god hypothesis?

You can only be truly an agnostic when there is contradictory evidence or positive evidence is incomplete. Not in the complete absence of evidence.

#2 Chris (Guest) on Thursday May 16, 2013 at 7:20am

The absence of evidence, is not the evidence of absence..

#3 Somite on Thursday May 16, 2013 at 7:22am

Sorry but it is or there would be no way to distinguish reality from fiction.

#4 aminfidel on Friday May 17, 2013 at 5:03am

great article. thanks.

#5 William Walsh (Guest) on Friday May 17, 2013 at 5:41am

On this question, I agree with Somite.

The burden lies with the one making the positive claim. Until there is evidence, the absence of evidence, while not evidence in and of itself, invalidates the positive claim. Especially when the positive claim regards the existence of a supernatural realm and an all knowing, all powerful, all loving entity about whom there is absolutely no evidence.

All statements that do not explicitly deny the existence of something for which there is no evidence are, at best, logical nonsense.

#6 William Walsh (Guest) on Friday May 17, 2013 at 6:01am

I will add this: the question about a face card in the next few cards is a non sequitur. There is abundant evidence to support the "probably" response: it's a deck of cards and there are face cards in a deck of cards.

Jillette didn't trick us. Kurtenbach is trying!

#7 Seth Kurtenbach on Friday May 17, 2013 at 6:52am

Thanks for the feedback, everyone.

The face card example is not a non-sequitur. You are correct that there is an abundance of evidence supporting the belief that one of the next few cards will be a face card, and we agree that there is no evidence supporting the belief that God exists.

However, Penn's claim is about the logical relationship between belief and knowledge. He says that the lack of knowledge implies the lack of belief. I demonstrate that this is false with the face cards example: it is a case where one lacks knowledge, but has a belief. It is a counterexample to his claim.

Theists may not have evidence supporting their belief, but that is a different issue. The issue here is whether admitting that you lack knowledge logically requires you to give up the belief. It does not, as shown by the face card example.

Hope this helps clear things up.

#8 Somite on Friday May 17, 2013 at 7:01am

Lack of knowledge requires giving up the belief. Is it valid to believe things in the absence of evidence? I posit not regardless of what future knowledge we may come across.

#9 maggiesaurus may on Saturday May 18, 2013 at 4:02am

Great read. I loved the death/murder analogy.
I think some of the commenters think that the question is whether it's a good idea to believe something without knowledge. They are not grasping that a person certainly can believe something even though there is no evidence for it (like God, life after death, etc.).

#10 CF Parrinello (Guest) on Sunday June 02, 2013 at 9:29pm

One needn't to answer a theistic question in the negative to be an atheist. You simply need to lack belief in god(s). The assertion that that is an agnostic is, as you point out, not in the domain of gnosticism/agnosticism at all, but once again the prism of belief. If one doesn't have it, they're an atheist. Penn's right here: if you self-identify as an agnostic, you're an atheist.

#11 Somite (Guest) on Sunday June 02, 2013 at 11:19pm

Do you feel the same way about Santa or werewolves?

#12 Person (Guest) on Monday June 03, 2013 at 1:27am

The relationship between knowledge and belief is not bidirectional, ie one implies the other but it is not implied by the other itself. Knowledge claims clearly imply knowledge, but neither a belief nor a lack of it are necessarily connected with knowledge claims.
I can say that I don't know whether or not Aliens exist, but my assumption that there might be more life out there has very little to do with whether I can know it to be true or not.
Of course, if I did know, then I'd have to settle with a definitive position, but that's not the case when you're any type of agnostic.

#13 Somite (Guest) on Monday June 03, 2013 at 5:06am

No need to complicate. There must be evidence for knowledge but not for belief.

The question should be phrased as thus. There is no evidence of life in other planets but we know is possible because there is life on earth.

#14 Matt (Guest) on Monday June 03, 2013 at 9:54am

"Evidence" for God depends entirely on what one's definition of God is. It's easy to say, "There is no God" when your definition of God is limited to things we have no evidence for.

But when you expand your definition of God to include scientifically observable and provable phenomena, the game of "proving" or "disproving" God becomes just a semantics game.

The scientist has scientific sounding jargon to describe what he sees.

The believer has a name for the source from which the things the scientist describes originates from.

Science vs. God is and has always been false dichotomy. It is an exercise in semantics only.


#15 Somite (Guest) on Monday June 03, 2013 at 10:27am

A scientific fact based on evidence is not equivalent to a religious belief. The former is a verifiable reality. There is no evidence there is anything beyond the natural world. An unnecessary hypothesis.




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