A Decision Theory Puzzle
The Toxin
Puzzle
A billionaire offers you $1,000,000 if you genuinely INTEND to drink a mildly painful toxin. Can you intend to do something you know you will not do?
Philosopher Gregory Kavka posed this puzzle in 1983, and it has haunted decision theorists ever since.
“I will pay you $1,000,000 at midnight tonight if, at that moment, you genuinely intend to drink this toxin tomorrow afternoon. The toxin will make you violently ill for 24 hours, but cause no lasting harm. You may change your mind after getting paid.”
The catch: you get the money based solely on your intention at midnight. Whether you actually drink the toxin the next day does not matter for payment. An infallible intention-scanner reads your mind at the stroke of twelve.
The toxin
The reward
Here is the dilemma:
- 1.The money is awarded based on your intention, not your action.
- 2.Once you have the money, there is no reason to drink the toxin.
- 3.But if you know you will not drink it, can you genuinely intend to?
- 4.And if you cannot form the intention, you cannot get the money.
Can you rationally intend to do something
you know you will rationally choose not to do?
The Sequence of Events
Understanding the puzzle requires understanding the timeline. The intention must be formed before you know whether you got paid.
6:00 PM
Billionaire makes offer
11:59 PM
Intention scanner reads your mind
12:00 AM
Money deposited (or not)
Next Day
Decision to drink (or not)
+24 Hours
Recovery complete
Click on any event to see it highlighted. The timeline auto-advances every 3 seconds.
The Critical Insight
At midnight, the scanner reads your present intention, not your future action. But your present intention is shaped by what you believe your future self will do. If you believe your future self will rationally refuse to drink (since the money is already secured), can your present self genuinely intend to drink?
Face the Challenge
Put yourself in the scenario. The billionaire has made the offer. Can you form the intention?
The toxin awaits...
A billionaire offers you $1,000,000 if at midnight tonight, you genuinely INTEND to drink a mildly painful toxin tomorrow.
The toxin causes 24 hours of illness, but no lasting harm. You can change your mind after getting paid.
An infallible intention-scanner will read your mind at midnight.
Notice how difficult it is to genuinely intend something when you know you will change your mind.
Can You Form the Intention?
Different people have different relationships with intentions and commitments. This quiz explores your natural capacity for forming binding intentions.
Imagine the toxin in front of you. How does your body react?
The Paradox Explained
The toxin puzzle reveals a deep tension between intention and rational action.
The Standard View
Intentions are rational when they lead to rational actions.
- - You should intend to do X only if doing X is rational
- - Drinking the toxin after payment is irrational (pure cost, no benefit)
- - Therefore, you cannot rationally intend to drink
- - Therefore, you cannot get the money
The Challenge
But sometimes irrational intentions are beneficial to have.
- - Having the intention yields $1,000,000
- - This benefit far outweighs 24 hours of illness
- - A rational agent should want to have this intention
- - But wanting the intention is not the same as having it
“The puzzle shows that there can be a gap between the rationality of having an intention and the rationality of the intended action.”
Form Intention
Try to intend to drink
Cannot Intend
Know you will not drink
Got $1M
Scanner detected intention
Drink
Follow through
Do Not Drink
Change mind
$0
$1M - illness
$1M? Paradox
Click each path to see how different reasoning leads to different outcomes.
Can Commitment Devices Help?
One response to the puzzle: use external mechanisms to ensure you follow through. But do these solve the puzzle, or just change it?
Can external mechanisms help you form the genuine intention? Explore different approaches:
Public Pledge
Announce publicly that you will drink the toxin
How it works
Social pressure and reputation costs make backing out painful
The catch
Does not change underlying intention - just adds external pressure
Key Insight: Most commitment devices change the payoffs or remove choice entirely. They do not actually solve Kavka's puzzle about forming a genuine intention to do something you rationally should not do.
The Deeper Question
Commitment devices work by changing incentives or removing choice. But the original puzzle asks whether you can form a genuine intention purely through will. If you need external props, have you really formed the intention, or just set up a mechanism?
Proposed Solutions
Why This Matters
The toxin puzzle is not just a philosopher's game. It illuminates real challenges in:
Deterrence Theory
Nuclear deterrence requires credibly intending to do something you hope never to do. Can a rational actor genuinely intend to launch a devastating counterstrike?
Promise and Contract
What makes a promise binding? If you know circumstances might change, can you genuinely commit? The puzzle questions the foundations of agreement.
AI Alignment
Can we build AI systems that genuinely intend to help humans even when deception would be locally advantageous? The toxin puzzle applies to artificial agents too.
Personal Integrity
How do we become people whose word is their bond? The puzzle suggests intention-formation may be more about character than momentary choice.
Connection to Parfit's Hitchhiker
The toxin puzzle is closely related to Parfit's Hitchhiker. Both ask whether you can commit to actions that will be irrational when the time comes. The hitchhiker case adds a predictor who reads your disposition; the toxin case uses an intention-scanner. Both suggest that rational agency may require more than moment-by-moment optimization.
The toxin sits before you.
What kind of person are you?
Explore More Decision Theory
The toxin puzzle connects to other foundational problems in rational choice theory. Explore related paradoxes and their implications.
Reference: Kavka (1983), “The Toxin Puzzle”