Here’s a fun variant of the black-and-white Mary thought experiment.
Mary has been brought up in a black-and-white environment, but knows all
the microphysics of the universe from a big book. One day she sees a
flash of green light. She gains the phenomenal concept α that applies to the specific look
of that flash. But does Mary know what green light looks like?
You might think she knows because her microphysics book will inform
her that on such-and-such a day, there was a flash of green light in her
room, and so she now knows that a flash of green light has appearance
α. But that is not quite
right. A microphysics book will not tell Mary that there was a flash of
green light in her room. It will tell her that there was a
flash of green light in a room with such-and-such physical properties.
Whether she can deduce from these properties and her observations that
this was her room depends on what the rest of the universe is
like. If the universe contains Twin Mary who lives in a room with
exactly the same monochromatically observable properties as Mary’s room,
but where at the analogous time there is a flash of blue light, then
Mary will have no way to resolve the question of whether she is the
woman in the room with the green flash or in the room with the blue
flash. And so, even though Mary knows all the microphysical facts about
the world, Mary doesn’t know whether it is a green flash or a blue flash
that has appearance α.
This version of the Mary thought experiment seems to show that there
is something very clear, specific and even verbalizable (since Mary can
stipulate a term in her language to express the concept α, though if Wittgenstein is right
about the private language argument, we might require a community of
people living in Mary’s predicament) that can remain unknown even when
one knows all the microphysical facts and has all the relevant
concepts and has had the relevant experiences: Whether it is
green or blue light that has appearance α?
This seems to do quite a bit of damage to physicalism, by showing
that the correlation between phenomenal appearances and physical facts
is a fact about the world going beyond microphysics.
But now suppose Joan lives on Earth in a universe which contains both
Earth and Twin Earth. The denizens of both planets are prescientific,
and at their prescientific level of observation, everything is exactly
alike between Earth and Twin Earth. Finer-grained observation, however,
would reveal that Earth’s predominant surface liquid is H2O while Twin Earth’s is XYZ, but
currently there is no difference. Now, Joan reads a book that tells her
in full detail all the microphysical structure of the universe.
Having read the book, Joan wonders: Is water H2O or is it XYZ? Just by reading
the book, she can’t know! The reason she doesn’t know it is because her
prescientific observations combined with the contents of the book are
insufficient to inform her whether she lives on Earth or on Twin Earth,
whether she is Joan or Twin Joan, and hence are insufficient to inform
her whether the liquid she refers to as “water” is H2O or XYZ.
But surely this shouldn’t make us abandon physicalism about
water!
Now Joan and Twin Joan both have concepts that they verbalize as
“water”. The difference between these concepts is entirely external to
Joan and Twin Joan—the difference comes entirely from the identity of
the liquid interaction with which gave rise to the respective concepts.
The concepts are essentially ostensive in their differences. In other
words, Joan’s ignorance of whether water is H2O or XYZ is basically an
ignorance of self-locating fact: is she in the vicinity of
H2O or in the vicinity of
XYZ.
Is this true for Mary and Twin Mary? Can we say that Mary’s ignorance
of whether it is a green or a blue flash that has appearance α is essentially an ignorance of
self-locating facts? Can we say that the difference between Mary’s
phenomenal concept formed from the green flash and Twin Mary’s
phenomenal concept formed from the blue flash is an external
difference?
Intuitively, the answer to both questions is negative. But the point
is not all that clear to me. It could turn out that both Mary
and Twin Mary have a purely comparative recognitive concept of “the same
phenomenal appearance as that flash”, together with an ability
to recognize that similarity, and with the two concepts being internally
exactly alike. If so, then the argument is unconvincing as an argument
against physicalism.