Demon (thought experiment)
- Descartes’ malicious demon – Cartesian skepticism (also called methodological skepticism) advocates the doubting of all things that cannot be justified through logic. Descartes uses three arguments to cast doubt on our ability to objectively know: The dream argument, the deceiving God argument, and the malicious demon argument.  Since our senses cannot put us in contact with external objects themselves, but only with our mental images of such objects, we can have no absolute certainty that anything exists in the external world. In the evil demon argument Descartes proposes an entity who is capable of deceiving us to such a degree that we have reason to doubt the totality of what our senses tell us.
- Laplace's demon is a hypothetical all-knowing entity (later called "Laplace's Demon") who knows the precise location and momentum of every atom in the universe, and therefore could use Newton's laws to reveal the entire course of cosmic events, past and future. Based upon the philosophical proposition of causal determinism.
- Maxwell's demon can distinguish between fast and slow moving molecules. If this demon only let fast moving molecules through a trapdoor to a container, the temperature inside the container would increase without any work being applied. Such a scenario violates the second law of thermodynamics.
- Morton's demon stands at the gateway of a person's senses and lets in facts that agree with that person's beliefs while deflecting those that do not. This demon is used to explain the phenomenon of confirmation bias.
- In aphorism 341 of The Gay Science, Nietzsche puts forth his eternal recurrence concept. In it, he employs a demon with special metaphysical knowledge as an agent for forcing reevaluation of perspective on one's own life.
- The Darwinian Demon is a hypothetical organism that can simultaneously maximize all aspects of its fitness.
There are other creatures that feature in thought experiments about philosophy. One such creature is a utility monster, a creature that derives much more utility (such as enjoyment) from resources than other beings, and hence under a strict utilitarian system will have more or all of the available resources directed to it. Newcomb's paradox supposes a being who is believed to be capable of predicting human behavior; Robert Nozick suggested a "being from another planet, with an advanced technology and science, who you know to be friendly".