Multiple realizability, in the philosophy of mind, is the thesis that the same mental property, state, or event can be implemented by different physical properties, states, or events. Philosophers of mind have used multiple realizability to argue that mental states are not the same as — and cannot be reduced to — physical states. They have also used it to defend many versions of functionalism, especially machine-state functionalism. In recent years, however, multiple realizability has been used to attack functionalism, the theory that it was originally used to defend. As a result, functionalism has fallen out of favor in the philosophy of mind.