A theory of ontology that there are only two real substances in the universe, one being material and the other being mental. Thus, the mind and body (or brain) exist independently of each other and have entirely different essences or natures. Historically, it is exemplified by the following distinctions: Pre-Socratic appearance-reality, Plato’s forms-world, Descartes’s mind-matter, Hume’s fact-value, Kant’s empirical-phenomena-transcendental noumena, Heidegger’s being-time, and Russell’s existence-subsistence. In quantum mechanics, dualism refers to the wave-particle duality: matter can exhibit both waves and particles as in the case of light and electromagnetic radiation, both of which were considered to be wave phenomena in classical physics (light has to be described as a wave in order to explain interference effects and the photoelectric effect is usually explained by involving the particle nature of light). Such dual descriptions may have valid overlaps and when they do dual theories do not give contradictory predictions
See Appearance-reality distinction, Mind-body problem, Monism, Motor learning, Ontology, Quantum mechanics