The thought experiment posits a friend of Wigner in a laboratory, and lets the friend perform a quantum measurement on a physical system (this could be a spin system or something analogous to Schrödinger's cat). This system is assumed to be in a superposition of two distinct states, say, state 0 and state 1 (or "dead" and "alive", in the case of Schrödinger's cat). When Wigner's friend measures the system in the 0/1-basis, according to quantum mechanics, they will get one of the two possible outcomes (0 or 1) and the system collapses into the corresponding state.