QuestionsIs the mind the brain?What is it like to be a bat?
Is the mind the brain?The mind is not matter

What is it like to be a bat?

Nagel's bat ("what it is like")

Thomas Nagel born 1937Consciousness is a subjective point of view that physics does not describe: our idea of nature must be widened.
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What it claims

An objective physical theory does not describe what it is like to be a bat (or to be us): part of the mind escapes it, and today we have no idea what a physical explanation of these facts would look like.

Why

What it is like to be someone depends on their viewpoint, and a physical theory steps away from every viewpoint.

The same idea through history
Gottfried Wilhelm Leibniz1714Leibniz already imagines walking into a thinking machine as into a mill: we would see parts pushing each other, never a perception. The idea that a mechanical description leaves out experience is very old.
Thomas Nagel1974Nagel makes the phrase "what it is like to be..." the mark of consciousness, and picks the bat as an example of a form of life alien to us.
Frank Jackson1982Frank Jackson turns the idea into a story: a scientist who knows all the physics of colour without ever having seen colour.David Chalmers1995David Chalmers takes up Nagel's phrase to define the "hard problem" of consciousness.
In short

"What is it like to be a bat?" is an argument by Thomas Nagel ("What Is It Like to Be a Bat?", The Philosophical Review, 1974, 1974) on the question "Is the mind the brain?" (answer: the mind is not matter). Its claim: An objective physical theory does not describe what it is like to be a bat (or to be us): part of the mind escapes it, and today we have no idea what a physical explanation of these facts would look like. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 3 premises. AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

"What Is It Like to Be a Bat?", The Philosophical Review, 1974
Valid

To know what an organ like the liver is, we can dissect and measure it. But for a consciousness something is missing: what it is like to live it from the inside. In 1974 Thomas Nagel takes the example of the bat, which perceives the world by echolocation, a sense we do not have. We can study its brain and its sonar in the finest detail; would we then know what it is like to be a bat? Nagel thinks not: this point of view is subjective, while science seeks an objective description, which is nobody's in particular. We list it here because the argument shows that the mind cannot be reduced to what physics describes today. Note that Nagel himself is not a dualist. He does not defend a soul separate from the body, and he writes that it would be a mistake to conclude that physicalism is false. He only says that we have no conception of how it might be true.

P1Bats have experiences: there is something it is like to be a bat, from its own point of view. That is what "having conscious states" means.
P2Every fact about "what it is like" to be someone is tied to a single point of view, that of the being who has the experience.
P3A physical theory is objective: it moves away from any particular point of view to describe things as they are for everyone.
P4 · implicit premiseWhat a theory gives up by moving away from a point of view, it does not describe.
So an objective physical theory does not describe what it is like to be a bat (or to be us): part of the mind escapes it, and today we have no idea what a physical explanation of these facts would look like.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P4, one could accept P1 to P3 and say that physics, while moving away from points of view, still describes the subjective facts in another way, as a map describes a landscape without being a landscape.
The argument is valid. P1 says there are facts about "what it is like". P2 ties them to a point of view. P3 says physics gives up particular points of view. With P4, these facts are therefore not described by physics. The debate is mostly about P1 (are there such facts over and above the rest?) and about the step from "our theory does not describe them" to "no physical theory ever could".
Analogy. A recipe describes a cake very precisely, but someone who has never tasted sugar does not learn what sweetness is like by reading it. The recipe is objective; the taste is not.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Daniel Dennett 1988 · attacks premise 1

The notion of a private, ineffable "what it is like" added to everything the brain does is confused. Dennett thinks the defenders of materialism were wrong to say "we will explain these qualia"; they should have asked "what qualia?". In his view there are no such facts over and above what we can discriminate, react to and say.

Nagel replies
David Chalmers 2003
Chalmers says that this kind of materialist must either deny even the appearance of a further fact to explain, which seems to deny the obvious, or give substantial arguments for why it is only an appearance.
The debate goes on (our summary) The debate goes on. Dennett answers that what seems obvious comes from a misleading way of describing our own inner life, and he proposes to study what people say about their experiences (heterophenomenology). His opponents think this changes the subject. Neither side has won.
Patricia Churchland 1996 · attacks the conclusion

That we cannot imagine how the brain produces experience is a fact about us, not about the world. Churchland judges that reasoning from our present ignorance, or from what we cannot imagine, to a limit on science is bad reasoning.

no reply
Nagel never answered this objection.
The debate goes on (our summary) Nagel himself says his argument does not prove physicalism false (see the next reply). The question is whether our present ignorance is a mere delay of science, as for other solved mysteries, or whether subjective experience is different from other successful reductions. Nagel thinks it is different because the past examples contained no subjective point of view; Churchland is not convinced.
Thomas Nagel (his own caveat) 1974 · attacks the conclusion

Nagel himself limits the reach of his argument: it would be a mistake to conclude that physicalism is false. He says it cannot be understood, since we have no conception of how it might be true. If mental processes are physical processes, then there is something it is like to undergo certain physical processes, and what that amounts to remains a mystery.

Nagel replies
Thomas Nagel 1974 · Same passage
Nagel writes that it would be truer to say that physicalism is a position we cannot understand, because we do not at present have any conception of how it might be true.
The debate goes on (our summary) This caution is why Nagel is not a dualist in the strict sense. Later he defended the idea that nature must contain more than what current physics describes (Mind and Cosmos, 2012), but this 1974 argument does not go that far.
What today's voices say
Defend it
Sam HarrisHe uses Nagel's definition: consciousness is 'the fact that it's like something to be a system'.