This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Thomas Nagel asked a question that sounds almost too simple to be a philosophical argument: what is it like to be a bat. Not what a bat's brain does, not how sonar works, not which neurons fire when a bat navigates in the dark, all of which a sufficiently patient physiology could in principle describe completely. Nagel's question is about the bat's own point of view, the felt quality of echolocating a moth from the inside, and his claim is that no amount of objective, third person description, however complete, delivers that. You can know everything physical there is to know about a bat and still not know what it is like to be one, because that knowledge is essentially a first person kind, tied to a particular point of view rather than freed from one. This is the philosophical vocabulary that has since collected under the word qualia, the felt, subjective quality of an experience, the redness of red or the specific ache of a headache, considered apart from whatever causal role it plays. A physicalist owes an account of why creatures with the right neural architecture have any felt quality at all rather than none, why there is something it is like to be a bat and, so far as anyone can tell, nothing it is like to be a thermostat, even though both are physical systems responding to their environment. That question, sometimes called the hard problem, is not a request for more neuroscience. It is a request for an explanation of why physical processing is accompanied by experience at all, and it is the reason Nagel's small, almost whimsical question about a bat still organizes a serious argument.