The argument, which shows up in various forms in numerous philosophical papers and thought experiments, is that we can have a complete physical understanding of a conscious being, but still not know how it feels to be that being. We can know everything about a bat’s nervous system, Thomas Nagel argues, but still not know what it’s like to be a bat. The usual implication is that there must be something non-physical involved.
Maybe the most famous thought experiment on this is Mary’s Room, which I posted about a few years ago. However, that post was in the context of sharing a video, and so was a bit cursory, although it sparked a good discussion at the time. My views, although still similar, may have clarified a bit, so it feels like time for a fresh take.
In case you’re not already familiar with it, here’s a quote of Frank Jackson’s original description via the Stanford Encyclopedia article on the subject: