In philosophy, the brain in a vat (BIV) is a scenario used in a variety of thought experiments intended to draw out certain features of human conceptions of knowledge, reality, truth, mind, consciousness, and meaning. Originated by Gilbert Harman, Hilary Putnam turned the scenario into a modernized version of René Descartes's evil demon thought experiment. Following many science fiction stories, the scenario involves a mad scientist that might remove a person's brain from the body, suspend it in a vat of life-sustaining liquid, and connect its neurons by wires to a supercomputer that would provide it with electrical impulses identical to those a brain normally receives. According to such stories, the computer would then be simulating reality and the "disembodied" brain would continue to have perfectly normal conscious experiences, such as those of a person with an embodied brain, without these being related to objects or events in the real world. According to Putnam, the thought of "being a brain-in-a-vat" (BIV) is either false or meaningless. Considered a cornerstone of Semantic externalism, the argument produced significant literature. The Matrix franchise and other fictional works (below) are considered inspired by Putnam's argument.

A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences. The concept is also referred to as a Gedankenexperiment within the work of Ernst Mach and includes thoughts about what may have occurred if a different course of action were taken as opposed to what did in fact occur. The importance of this ability is that it allows us to imagine what may occur in the future, as well as the implication of alternate courses of action.