Some would conclude that this thought experiment, like many others, depends
so precariously on linguistic hair-splitting – what counts as ‘knowing’ or ‘learning’,
what do we mean by ‘physical information’, or indeed ‘all’? – that we can only ever
get out of it what we put in. Others maintain that Mary can be helpful in making
a tricky dichotomy easier to think about. If you believe that Mary will be surprised
when she comes out, then you probably believe that consciousness, subjective
experience, or qualia are something additional to knowledge of the physical
world. If you think she will not be surprised, then you likely believe that knowing
all the physical facts tells you everything there is to know – including what it is like
to experience something.
THE PHILOSOPHER’S ZOMBIE
Imagine there is someone who looks like you, acts like you, speaks like you, and
in every detectable way is identical with you, but is not conscious. An early form
of this idea was the ‘zombie replica’, an exact physical replica of a man, ‘to which
there applied only the physical descriptions and whatever they entailed’ (Kirk
and Squires, 1974, p. 141), behaviour being one of
the aspects entailed (Kirk, 2005). There have been
many variations on this theme, but we will stick with
the popular version proposed by Chalmers (1996), a
creature that is indistinguishable from a conscious
one. There is nothing it is like to be this creature.
There is no view from within. No consciousness. No
qualia. This – not something from World War Z or The
Walking Dead – is the philosopher’s zombie.
This zombie has caused even more trouble than
Mary. Most people agree that it is easy to imagine
one, but is a zombie either logically or physically
possible?
Chalmers thinks so: ‘the logical possibility of zombies
seems [. . .] obvious to me. A zombie is just something
physically identical to me, but which has no conscious
experience – all is dark inside’. He goes on, ‘I can detect
no internal incoherence; I have a clear picture of what
I am conceiving when I conceive of a zombie’ (1996,
pp. 96, 99). Chalmers’s zombie twin, living on zombie
earth, is quite conceivable, he argues. He suggests we
imagine a silicon version of Chalmers who is organ-
ised just like the real philosopher and behaves just
like him but has silicon chips where the real one has
neurons. Many people would expect such a creature
to be unconscious (whether or not it would be). Then,
he suggests, just replace the chips with neurons, and
you have my zombie twin – totally indistinguishable
from the real philosopher, but with nothing it is like to
be him. This works, he argues, because there is noth-
ing in either silicon or biochemistry that conceptually
entails consciousness.
‘conscious states are
qualitative states right
down to the ground.
Take away the qualia and
there is nothing there.’(Searle, 1998, p. 21)‘A zombie is just something
physically identical to
me, but which has no
conscious experience – all
is dark inside’(Chalmers, 1996, p. 96)PRoFILe 2.1
David Chalmers (b. 1966)
Born in Australia, David
Chalmers originally intended
to be a mathematician, but
then he spent six months
hitchhiking around Europe on
his way to taking up a Rhodes
scholarship at Oxford, and spent most of his time thinking
about consciousness. This led him to Douglas Hofstadter’s
research group and a PhD in philosophy and cognitive
science. He is responsible for the distinction between the
‘easy problems’ and the ‘hard problem’ of consciousness,
and he is one of that rare breed: a self-proclaimed dualist.
As well as helping to get a science of consciousness off the
ground, his other interests include artificial intelligence
and computation, philosophical issues about meaning
and possibility, and the foundations of cognitive science.
Working for many years in Tucson, Arizona, he founded
the ‘Toward a Science of Consciousness’ conferences. He
is now Distinguished Professor of Philosophy and director
of the Centre for Consciousness at the Australian National
University in Canberra, as well as Professor of Philosophy
and co-director of the Center for Mind, Brain, and Con-
sciousness at New York University.