Can you ever really know what it's like to be a bat?

Not fully, argued the philosopher Thomas Nagel, though not everyone agrees. In a 1974 paper, Nagel granted that you can imagine flying, navigating by sonar, hanging upside down and eating insects. But that only tells you what it would be like for you to behave like a bat, not what it is like for the bat. Every experience, he argued, is tied to a single point of view, and you can't step into someone else's.

The idea came from home: Nagel lived in a house where bats were frequent visitors. He chose them because they perceive the world through echolocation, a sense very different from ours, even though it plays a role similar to vision. Even if you gradually turned into a bat, he argued, your brain wouldn't have been wired as a bat's from birth, so you'd live a bat's life without its mindset. Daniel Dennett, who disagreed with him sharply, still called the paper the most widely cited and influential thought experiment about consciousness.

Nagel's real target is bigger than bats. An organism has conscious mental states, he wrote, if and only if there is something it is like to be that organism. That phrase became the standard way to talk about consciousness. His point is that this subjective character of experience can't be translated into purely physical descriptions without losing the very thing you meant to explain.

Critics push back. Dennett argued that the interesting features of a bat's experience can be studied from outside: we know, for instance, that bats can't detect objects more than a few meters away, because echolocation has a limited range. Eric Schwitzgebel and Michael Gordon went further, arguing that sighted humans echolocate much like bats, just without noticing.