The dominant tendency in philosophy of mind through the 1960s was to explain mental states in physical or functional terms: pain is a certain state of a nervous system, or a certain input-output relation. Thomas Nagel's short paper of 1974 showed as concretely as possible what that programme leaves out.

The choice of the bat is not incidental: a bat is a mammal, close enough to us that we do not doubt it is conscious, yet it perceives the world by sonar — a sense with no counterpart in us. Nagel's argument is that even if we mapped the bat's nervous system completely and knew the physics of echolocation in full, we would not thereby know *what it is like to be a bat*. We can imagine ourselves behaving as a bat does, but that is a human imagining bathood. Because subjective experience is tied to a point of view, it is closed to an objective scientific description that aims to be independent of every point of view.

Nagel does not present this as a defence of dualism or a refutation of physicalism; he says we do not yet know what physicalism even means. The paper set the agenda for the next fifty years in philosophy of mind: Frank Jackson's knowledge argument of 1982, the 'hard problem' David Chalmers formulated in 1995 and the debates over qualia all run in the space it opened. Critics such as Daniel Dennett argue that the question itself misleads. In a period when artificial systems match humans at the level of language and behaviour, the question has become practical: how could we tell from outside whether a system is conscious?