The philosopher Thomas Nagel drew popular attention to the Hard Problem four decades ago in an influential essay titled “What Is It Like to Be a Bat?” Frustrated with the “recent wave of reductionist euphoria,” Nagel challenged the reductive conception of mind—the idea that consciousness resides as a physical reality in the brain—by highlighting the radical subjectivity of experience. His main premise was that “an organism has conscious mental states if and only if there is something that it is like to be that organism.”
If that idea seems elusive, consider it this way: A bat has consciousness only if there is something that it is like for that bat to be a bat. Sam has consciousness only if there is something it is like for Sam to be Sam. You have consciousness only if there is something that it is like for you to be you (and you know that there is). And here’s the key to all this: Whatever that “like” happens to be, according to Nagel, it necessarily defies empirical verification. You can’t put your finger on it. It resists physical accountability.