Sleeping Beauty Problem

What is the Sleeping Beauty Problem?

The Sleeping Beauty Problem makes us think hard about chances (probability) and what we believe about our place and time in the world (self-locating belief). Imagine a story where someone, called Sleeping Beauty, is in a strange science test.

In this puzzle, made in the year 2000 by a thinker named Adam Elga, Sleeping Beauty signs up for an odd test. On the first day, which is a Sunday, she falls asleep. They then toss a fair coin that can either land on heads or tails. If the coin shows heads, Sleeping Beauty wakes up on Monday. But if the coin shows tails, she wakes up both on Monday and Tuesday. She’s given a medicine each time she wakes up that makes her forget she was awake. This way, she can’t tell if it’s Monday or Tuesday. They tell her about all these steps before the test starts.

The big question of the Sleeping Beauty Problem is: When Sleeping Beauty wakes up during the test and is asked, “What’s the chance that the coin shows heads?” what should she say?

Key Arguments

  • The Thirder Position: People who think this way say that when Sleeping Beauty wakes up, the chance the coin shows heads is 1 out of 3. They explain it this way: There are three possible times she could wake up – Monday with heads, Monday with tails, or Tuesday with tails. Since she doesn’t know which waking up it is, she should think each time is equally possible.
  • The Halfer Position: Others believe the chance of heads is still 1 out of 2, just like it was before she went to sleep. They argue that the coin toss is just one event with two possible outcomes, and it doesn’t matter how many times Sleeping Beauty is woken up, it doesn’t change the first chance.

Both sides have strong reasons, and it’s good to remember that this isn’t just about numbers and chances but also how we deal with not knowing things and the way we see time and ourselves.

Answer or Resolution (if any)

Right now, there’s no answer that everyone agrees on for the Sleeping Beauty Problem. Philosophers are split between Thirders and Halfers, and some have other ideas too. Some suggest a middle answer, others think we might solve it by looking at chance in a new way, and some even wonder if the whole puzzle is set up right.

Some thinkers say we should make the puzzle clearer or change the rules for how Sleeping Beauty makes her guess. But unless we learn something big and new about chance or someone gives a very strong reason that changes most people’s minds, the Sleeping Beauty Problem will probably stay without one answer that everyone thinks is right.

Major Criticism

There’s been some harsh talk about the Sleeping Beauty Problem. Some say it uses chance in the wrong way in situations where it doesn’t fit. Critics think that in cases where we’re trying to figure out where we are or what time it is, regular ideas of chance can’t handle the tricky parts of what we feel or experience. Also, there’s a complaint that the puzzle isn’t clear enough or mixes up different kinds of unsureness – one about a real happening (the coin toss) and one about what a person goes through (the awakening).

Practical Applications (if any)

  • Anthropic Reasoning: The Sleeping Beauty Problem is useful in “anthropic reasoning.” This is when we think about how the fact that we exist could affect our view of chances. It’s helpful in thinking about big questions in space and deep thoughts, where the place or time someone is in could change the chance of seeing something.
  • Decision Theory: It might help when we need to make decisions without being sure of what will happen, especially when the results have to do with where or when we are, like betting or investing.
  • Artificial Intelligence: Machines that learn from what happens around them or change what they think might be affected by this debate about chances when it comes to where or when we think we are.

Each of these uses of the problem deals with thinking about chances when where or when we are might change what we think chances mean.

Related Topics with Explanations

  • Bayesian Probability: This is a way of thinking about chance that uses past events to guess the chance of something happening in the future. It is closely linked to the Sleeping Beauty Problem because it deals with updating our beliefs as new information comes in.
  • Quantum Mechanics: In the tiny world of atoms and particles, things can happen that are hard to guess. Quantum mechanics sometimes uses ideas like those in the Sleeping Beauty Problem when we try to describe how we see things happen in that world.


The Sleeping Beauty Problem is not just a puzzle; it’s a challenge that makes us question our belief in chance and how we understand ourselves and the world. Whether we agree with Thirders, Halfers, or someone else, this debate is more than just talking – it’s about serious things, like how we make decisions and how we face things we can’t be sure about, wrapped up with our own experiences. For now, the Sleeping Beauty Problem continues to create important discussions among thinkers, and it has big effects on the ways we figure out how to see our place in the big picture. It’s not just about the numbers; it’s about how the things that are sure and set in the outside world connect with the things we feel and experience inside us.