What is it like to be a bat?
Physicalism—which claims everything in the universe is physical—should be able to explain subjective experiences. But it seems impossible that anything we might learn about bat neurophysiology could shed light on what it is like to be a bat.
Imagine what it would be like to be a bat, perceiving your environment through echolocating sonar, eating insects, and flying through the night. It seems impossible that person could ever experience what a bat experiences.
At best, one can guess what it would be like to use echolocation, but one can never really know what it is like to be a bat.
Thomas Nagel (1974).