What is it like to be a bat?
In his famous essay, “What is it like to be a bat?” Thomas Nagel argues that many attempts to explain consciousness are too reductionist. He argues that they fail to account for the subjective character of experience, what he refers to as qualia.
He suggests that “an organism has conscious mental states if and only if there is something that it is like to be that organism - something it is like for the organism.” This is what he calls the subjective character of experience. To say that someone or something is conscious is to say that there is something it is like to be that person/thing. Most of us would probably agree that there isn’t something it is like to be a rock, but that there is something it is like to be a cow.
Nagel argues that we can’t possibly understand the subjective character of experience simply by examining the physical processes of the brain. Even with our own minds and brains, knowledge that brain state X correspond to subjective experience Y doesn’t provide an account of what it is like to experience Y. To illustrate this, he uses the example of a bat. Bats experience the world through echolocation, a sense that is fundamentally different from human vision.
While we can imagine what it would be like to act like a bat, we can’t grasp what it is like to experience the world through echolocation. In a sense, all we’re doing is mapping our own experience of consciousness onto the compatible aspects of a bat’s anatomy. We can probably imagine what it’s like to fly, and since both bats and humans have eyes, we can imagine the view from flight.
Nagel isn’t arguing against the existence of a relationship between the mind (consciousness) and the brain, nor that the source of consciousness must be supernatural. He’s simply suggesting that the nature of subjective experience may never be fully knowable, especially through a reductionist lens.
Qualia
Philosophers of mind use the term qualia to refer “to the introspectively accessible, phenomenal aspects of our mental lives.” In other words, qualia are subjective, qualitative states - they are the “what it is like” to have a certain experience like seeing the color red, feeling angry, or smelling a rose.