Think: A Compelling Introduction to Philosophy (11 page)

This would be what is called a psycho-physical identity theory.

Opponents sometimes say that you can only believe this theory
at the cost of feigning permanent anaesthesia. The complaint is
that everything distinctively mental has been left out. The correct rebuttal to this is to ask the challenger just what he thinks has been
left out, and watch him squirm on the difficulties of dualism. But
there are other difficulties in front of this kind of psycho-physical
identity theory. One is that in the case of mental events, one's own
consciousness rules, in the following sense. From the subject's perspective, anything that feels like pain is pain. It doesn't matter if it
is C-fibres, or something quite different. If someone had a minitransplant, in which organic C-fibres were replaced by something
silicon, for example, then if the silicon brings about the same results, it is still pain. Our knowledge of our pain is not hostage to the
question of whether we have C-fibres inside us, or any other particular kind of biological engineering. There is a first-person authority. Equally, although we might know whether marginal
candidates for feeling pain, such as perhaps shrimp, do or do not
have C-fibres, we might be uncomfortable in declaring them to
suffer pain or not purely on that account. So the identity does not
seem quite so straightforward as in other scientific cases (this
could be challenged).

We would be pleased enough if we could come to see the relation
between mental events and events in the brain or body as clearly as
we can see the relation between temperature and mean kinetic energy in gases. Perhaps it would not matter much to us whether the
result was achieved more by `pure thought, or more by experiment. So we can appreciate Leibniz's objection to Locke without
entirely sharing his rationalism. Still, when we try to think hard
about the relationship between brain and body on the one hand
and mind on the other, it usually seems to he our thinking rather
than mere scientific ignorance that is letting us down. Recently many scientists have turned their attention to consciousness, and a
variety of brain states have been identified as implicated in normal
conscious functioning. For example, electromagnetic waves in the
brain of a particular low frequency have been thought to be vital.
But it is not clear that this kind of truth is adapted to solving the
problem-to enabling us to side with Leibniz against Locke. From
the Lockean point of view, all the scientist may have discovered is
that when the brain is in some specific state, we get symptoms of
consciousness. But that might just tell us what consciousness is annexed to, by happenstance. It does not make the combination intelligible. And it also presupposes a right to shove the Zombie and
Mutant possibilities out of sight, for otherwise the scientist could
never establish the correlation, except at best in his or her own case.
But according to new mysterians, neither science nor philosophy
will ever get us to a point where things are better. We will never be
able to side wholeheartedly with Leibniz against Locke.

INVERTED SPECTRA: PRIVATE
LANGUAGES

The case of colour often seems especially to open wide the possibility at least of Mutants-people physically identical who nevertheless perceive colours quite differently. There might even be
Mutants whose colour spectra are completely inverted with respect to each other, so that the experience one gets from light at the
red end of the spectrum is the very experience that the other gets from light at the blue end. And there would be nothing to tell them
that this is so.

Cartesian dualism opens the possibility of Zombies and Mutants. But perhaps it also opens an even more frightening possibility. If we think in the dualist way, we are apt to feel secure that at
least we know what our own experience is like. The minds of others may he a hit conjectural, but our own minds are well known to
us. But is even this true? Consider now not the minds of others, but
your own past experience. Are you sure that the world looks to you
today the same colour as it looked yesterday? Are you in fact sure
that it looked any colour yesterday-in other words, that you actually received the conscious experience that you remember yourself
as having had?

By asking these questions you are applying the Zombie and Mutant possibilities to your own past. Now of course, at first sight the
possibilities are even more outlandish and absurd than applied to
other minds. And we are inclined to retort that of course we know
perfectly well that colours looked much the same yesterday as they
do today. We would surely notice it if we woke up and the sky now
looked like grass did yesterday, and vice versa.

I agree of course that we would notice the change. But is this security guaranteed, given Cartesian dualism? It depends on what we
think about memory and mental events. Why should we be sure
that mental events-thought of as entirely distinct, remember,
from anything physical-leave reliable traces in memory? I can
check that my memory of the physical world is reliable enough. I
remember putting the car in the garage, and lo and behold, when I
go down, there it is. I remember the way to the kitchen, and lo and behold, get there without any effort or any mistake. But what
would check that my memory of the mental world is accurate? In
Locke's terms, why should it not be `God's good pleasure' to annex
certain mental modifications to me today, together with the delusive memory that similar ones were annexed to me yesterday?
Wittgenstein said:

Always get rid of the idea of the private object in this way: assume that it constantly changes, but that you do not notice the
change because your memory constantly deceives you.

This is the heart of the `anti-private-language' argument in his
Philosophical investigations (published posthumously in 1953), one
of the most celebrated arguments of twentieth-century philo-
sophy.Wittgenstein tried to show that there could be no significant
thought about the nature of one's past (or future) mental life if that
mental life is divorced from the physical world in the way that
Cartesian dualism proposes. It becomes, as it were, too slippery or
ghostly even to bean object of our own memories or intentions.

The Mutant and Zombie possibilities, applied to our own pasts,
are certainly unnerving. But really they ought only to unnerve us
about the dualist picture. Once more, can we recoil from Locke to
some version of Leibniz? Leibniz, remember, wants there to be a
`rational' relationship between the physical and the mental, so that
the mental event of seeing a colour is some kind of rational expression of what is going on physically, not an accidental annexation to
it. How could this work in the case of colours? The Leibnizian idea
is that if I and my twin (which now might be myself as I was yesterday) are functioning physically in the same way, then there is no possibility that our mental lives are different. How can we flesh out
this suggestion? Here is a sketch of an answer.

Many of the physical changes underlying colour perception are
fairly well understood. Colour perception is the result of the stimulation of the cones that pack the central part of the retina. The
current best theory suggests that there are three different kinds of
cone, L, M, and S (long, medium, and short). L cones `spike' or send
messages down the optic nerve more readily when light of longer
wavelength hits them, M cones get excited more when light of
medium wavelength does, and S cones when light of shorter wavelength does. The colour we perceive then depends in the first place
on a comparison between the levels of excitation of these three
kinds of cone. So, for instance, if S is much more excited than L this
codes for blue, the colour at the short wavelength end of the spectrum. If L is much more excited than S, this codes for yellow. If L is
more excited than M we get red, and if M is more excited than L, we
get green. It is as if the channels are `opponents' and the result depends on which of the opponents overcomes the other.

Now consider the fact that colours have a lot of interesting properties. Here are some: you cannot have a surface that is yellowy
blue. You can't have one that is reddish green. You can on the other
hand have surfaces that are bluish green, or yellowish red (orange).
You can't have a bright brown. You cannot have a bright grey (it is
difficult to imagine a grey flame or a brown flame). Yellow is a
lighter colour than violet. You can have a transparent red or blue or
green gem, but you cannot have a transparent white gem-the
nearest would be a milky white, like an opal. You can have white
light, but not black light.

All these might seem to be brute facts about the Cartesian realm
of the mind, where colours are supposed to hold their residence.
But we can begin to see them as expressions of various physical
facts. We can't see a surface as yellowy blue, because yellow and
blue are produced by mathematical opposites: we get yellow when
1, > S, and blue when Similarly for red and green. We cannot
have bright brown, because brown is darkened yellow. A surface is
seen as brown when it would be coded for yellow, except that there
is only a low overall energy level compared with that of other
sources of light in the context. Similarly forgrey, which is darkened
white. Yellow is lighter than violet because yellow light (I. > S) is
also nearer the frequency at which our visual systems are maximally responsive. By comparison both red at one end and blue at
theotherend of the visual spectrum are taking us towards the dark,
where we cannot respond at all. You cannot have transparent white
because something is only seen as white when it scatters light.

All this of course only scratches the surface of colour science.
But it gives us a glimmering at least of the way in which things
`make sense. With enough facts of this kind in front of us we might
be less enchanted by the inverted spectrum possibility. Let us take
first the simpler case of monochromatic (black-and-white) vision.
Suppose it is suggested that someone might he a physical duplicate
of me, but see as dark what I see as light, and vice versa. Is that possible? Our snap judgement might be that it is. Perhaps we imagine
the world appearing to him as it appears in a photographic negative. But this does not really work. If I make a piece of grey glass
lighter, I see better through it; if I make it darker, I see less well
through it. Since he is a physical duplicate, this has to he true of my twin. But for him, when we clear the glass it `seems' as though we
added soot, since it becomes subjectively darker. And when we add
soot it `seems' as though it is becoming clearer. But then we have to
imagine that for him, as a plate of glass becomes darker he sees
through it better and better, and as it becomes lighter he sees
through it worse and worse. And that just doesn't seem to make
sense. It doesn't mark a coherent possibility.

Now consider someone who is physically identical with me, but
supposedly sees yellow as I see blue, and vice versa. It is now not
quite so easy to imagine him. He has to respond in the same way as
I do, so he cannot go round saying that yellow is a dark colour, for
example. That difference in response and behaviour would be a
physical difference. So we have to ask how he sees blue as bright,
and yellow as dark. If he really sees yellow as dark, as I see blue, how
does he see brown? How does he see orange? Brown is darkened
yellow, but for him yellow is already dark. So it is difficult to imagine how his physical discriminations could match mine, given this
complete disparity in mental experience.

In short, the possibility becomes a good deal less clear, and we
may feel our way to denying that it is a possibility at all. We would
he engineering a conception of the mind that closes the gap between the physical and the mental, that is, between the fully functioning and responsive visual system in the brain and the
apparently superadded `subjective' qualia of colour experience.
Such a piece of engineering would be a vindication of Leibniz's position. Subjective colour experience becomes not just a queer addon, but the inevitable, rationally explicable, expression of the kinds
of physical functioning of the creatures that we are. If the same can be done for all the elements of our consciousness, the problem is
solved.

THOUGHT