The qualia objection

1    Physicalism

Physicalism is the thesis that, at bottom, the world is purely physical.  Or to put it slightly differently.
 
 
 

A complete specification of the physical facts is a complete specification of all the facts (including all the mental facts).

The physical facts are all the facts there are.

Physicalism entails:
 
 

If you know all the physical facts, you know all the facts.

1    Bats

If you knew all the physical facts about bats and their brains, you would know all there is to know about bats.

2    Colors
 

If you knew all the physical facts about color and color vision, you would know all there is to know about colors.

2    What is it like to be a bat?

In a famous paper, entitled "What is it like to be a bat?" available on-line, Thomas Nagel argued that even if one knew everything about the bat's brain and its physiology there is still something that one would not know: namely, what it is like to be a bat.  One would not know what it is like to experience the world as a bat experiences it.  One would not know, for example, what it is like to navigate the world by sound.  Further, if you don't know what it is like to be a bat, then there are facts about bats that you don't know.  Here is the argument.
 

Premise 1    If Physicalism is true then if you knew all the physical facts about bats and their brains, you would know all there is to know about bats.

Premise 2      You could know all the physical facts about bats and their brains,  and still not know what it is like to be a bat.

Premise 3    If you don't know what it is like to be a bat, there is something about bats that you don't know.
 

 
Conclusion: Physicalism is false.

3    What did Mary learn?

      "Mary is confined to a black-and-white room, is educated through black-and-white books and through lectures relayed on black-and-white television. In this way she learns everything there is to know about the physical
nature of the world. She knows all the physical facts about us and our environment, in a wide sense of 'physical'
which includes everything in completed physics, chemistry, and neurophysiology, and all there is to know about
the causal and relational facts consequent upon all this, including of course functional roles. If physicalism is
true, she knows all there is to know. For to suppose otherwise is to suppose that there is more to know than
every physical fact, and that is just what physicalism denies. Physicalism is not the noncontroversial thesis
that the actual world is largely physical, but the challenging thesis that it is entirely physical. This is why
physicalists must hold that complete physical knowledge is complete knowledge simpliciter.... It seems, however,
that Mary does not know all there is to know. For when she is let out of the black-and-white room or given a
color television, she will learn what it is like to see something red, say. This is rightly described as
learning--she will not say 'ho, hum.' Hence, physicalism is false. This is the knowledge argument against
physicalism in one of its manifestations."

More formally:
 
 

Premise 1    If Physicalism is true then if a person knew all the physical facts about color and color vision, she would know all there is to know about color.

Premise 2      Before Mary leaves the room she knows all the physical facts about color and color vision, but after she leaves the room she learns something new about colorówhat it is like to see the color red.

Premise 3    If you learn something new about color then you didn't previously know all there is to know about color.
 

 
Conclusion: Physicalism is false.

4  Are these new facts?

How plausible is the claim that Mary learns no new fact? What is the new fact that she learns? 

That redness looks like this!  (Is that a fact about redness?  In what sense is that a fact about redness?  Might redness have looked some other way?  Like this for example?