What is Newcomb's paradox in simple terms?

A nearly infallible predictor has already put money in a box based on what it expects you to do. Should you take only the box that might hold a fortune, or both boxes since their contents are already fixed? Both answers seem compelling.

Is there a correct answer?

There is no consensus. Evidential decision theory, which focuses on what an act indicates, recommends one box, while causal decision theory, which focuses on what an act brings about, recommends two, and the debate continues.

Newcomb's Paradox

🔬 Natural Sciences By CampusDown Wiki Editorial Team Last updated October 27, 2026

Newcomb's paradox is a thought experiment in decision theory devised by the American physicist William Newcomb and popularized by the philosopher Robert Nozick in a 1969 paper. A highly reliable predictor has already put $1 million in an opaque box or left it empty depending on what it predicted you would do, and two plausible principles of rational choice give opposite answers on whether to take only that box or also a transparent box holding $1,000.

Contents

1. The setup
2. Clashing principles and decision theory
3. The prisoner's dilemma and empirical work
4. FAQ
5. References

The setup

In front of you are a transparent box A containing $1,000 and an opaque box B. If the predictor foresaw that you would take only B, it put $1 million in B; if it foresaw that you would take both, it left B empty. The boxes have already been filled, and the predictor has almost never been wrong. Introducing the problem, Nozick (1969) wrote that to almost everyone it is perfectly clear and obvious what should be done, the difficulty being that people divide almost evenly on the problem. Gardner (1973) brought it to a wide audience in his Scientific American column.

Clashing principles and decision theory

The expected utility argument says one-boxers almost always get $1 million, so you should take one box. The dominance argument says the contents are already fixed, so whatever is in B, taking both boxes yields $1,000 more, and you should take two. The clash split decision theory in two. Evidential decision theory, in the spirit of Jeffrey's (1965) framework, asks whether an act is evidence of good outcomes and recommends one box; causal decision theory, developed by Gibbard and Harper (1978), Lewis (1981) and Joyce (1999), asks whether an act causes good outcomes and recommends two. Eells (1982) argued through the tickle defense that evidential theory can also recommend two-boxing, while Ahmed (2014) defended evidential decision theory.

The prisoner's dilemma and empirical work

Lewis (1979) argued that a prisoner's dilemma against someone you believe reasons like you is in effect a Newcomb problem, and Campbell and Sowden (1985) collected papers on both as paradoxes of rationality and cooperation. In psychology, Quattrone and Tversky (1984) reported that people treat their own actions as diagnostic signals of desirable states, as in the voter's illusion, and Shafir and Tversky (1992) found that many participants choose one box in Newcomb-type tasks. In Bourget and Chalmers's (2014) survey of philosophers, two-boxing was somewhat more popular than one-boxing.

FAQ

What is Newcomb's paradox in simple terms?: A nearly infallible predictor has already put money in a box based on what it expects you to do. Should you take only the box that might hold a fortune, or both boxes since their contents are already fixed? Both answers seem compelling.
Is there a correct answer?: There is no consensus. Evidential decision theory, which focuses on what an act indicates, recommends one box, while causal decision theory, which focuses on what an act brings about, recommends two, and the debate continues.

Sources reviewed: 2026-10-26

References

Nozick, R. (1969). Newcomb's problem and two principles of choice. In N. Rescher (Ed.), Essays in Honor of Carl G. Hempel (pp. 114–146). Dordrecht: Reidel.
Gardner, M. (1973). Mathematical games: Free will revisited, with a mind-bending prediction paradox by William Newcomb. Scientific American, 229(1), 104–109.
Gibbard, A., & Harper, W. L. (1978). Counterfactuals and two kinds of expected utility. In C. A. Hooker, J. J. Leach, & E. F. McClennen (Eds.), Foundations and Applications of Decision Theory (pp. 125–162). Dordrecht: Reidel.
Lewis, D. (1979). Prisoners' dilemma is a Newcomb problem. Philosophy & Public Affairs, 8(3), 235–240.
Lewis, D. (1981). Causal decision theory. Australasian Journal of Philosophy, 59(1), 5–30.
Jeffrey, R. C. (1965). The Logic of Decision. New York: McGraw-Hill.
Eells, E. (1982). Rational Decision and Causality. Cambridge: Cambridge University Press.
Campbell, R., & Sowden, L. (Eds.). (1985). Paradoxes of Rationality and Cooperation: Prisoner's Dilemma and Newcomb's Problem. Vancouver: University of British Columbia Press.
Joyce, J. M. (1999). The Foundations of Causal Decision Theory. Cambridge: Cambridge University Press.
Shafir, E., & Tversky, A. (1992). Thinking through uncertainty: Nonconsequential reasoning and choice. Cognitive Psychology, 24(4), 449–474.
Quattrone, G. A., & Tversky, A. (1984). Causal versus diagnostic contingencies: On self-deception and on the voter's illusion. Journal of Personality and Social Psychology, 46(3), 237–248.
Ahmed, A. (2014). Evidence, Decision and Causality. Cambridge: Cambridge University Press.
Bourget, D., & Chalmers, D. J. (2014). What do philosophers believe? Philosophical Studies, 170(3), 465–500.

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This article was last updated on October 27, 2026. It is based on widely recognized original works and textbooks; when citing it in a paper, please check the original sources listed in the references.