Free Think: A Compelling Introduction to Philosophy by Simon Blackburn

is a Zombie or a Mutant. Although his or her physical
self is just like yours, it would he an arbitrary exercise of God's
bounty to make their mental life similar as well. This is especially
obvious on the `occasionalist' version of the view: perhaps for his
own inscrutable reasons God treats my stubbing my toe as an occasion on which to insert pain into my mental biography, but not
so for you. On the other hand, if Leibniz is right, there is no such
possibility. If you and your twin both stub your toes with the same
force, and react physically in the same ways, then the`expression'of
the physical events in your minds must also be the same, just as the figures projected by two identical shapes on a plane at an angle
must be the same.

    It is interesting that Leibniz uses a mathematical analogy. It is
not just that he was an even better mathematician than Descartes,
and amongst other things invented the calculus. It is rather that for
Leibniz the whole order of nature must eventually be transparent
to reason. When things fall out one way or another it is not just that
they happen to do so. There must be, if we could only see far
enough, a reason why they do. Things have to make sense. When
Leibniz says God does nothing in an arbitrary or unprincipled way
he is not really expressing a piece of theological optimism, so much
as insisting that we ought to be able to see why things are one way
or another. This is his `principle of sufficient reason'. In Descartes's
terms, we ought to be able to achieve a clear and distinct idea of
why things fall out as they do. We should be able to gain insight into
why the way things are is the way they must be. It is this confidence
in what ought to be possible to reason that makes Leibniz, like
Descartes, a 'rationalist'.
    In the philosophy of mind the Leibnizian must deny the possibility of Zombies and Mutants. If the physical biography is fixed,
then the mental biography is fixed thereby. There is no independent variation, actual or possible. The philosophical problem is that
of understanding why this is so. It is a question of how to understand the way in which the entire physical story makes true the
mental story.
    Locke thought he could leave it open whether it is an immaterial `thing' (a ghost) within us that does the thinking, or whether it
is the physical system itself, since God can superadd thought to anything he likes. But he is abundantly clear that it takes a mind to
make a mind. It takes a special dispensation: thought cannot arise
naturally (or, as Leibniz has it, in a rationally explicable way) from
matter.

    For unthinking particles of matter, however put together, can
have nothing thereby added to them, but a new relation of position, which it is impossible should give thought and knowledge to them.
    It is this kind of a priori certainty about what can and cannot cause
other things that marks Locke, like everyone else of his time, as
fundamentally a rationalist, albeit one who is more nervous about
our powers of reason than Descartes and Leihniz.
    Thinkers about mind and matter have not got much beyond
Locke and Leihniz. Today as well there are thinkers (sometimes
called `new mysterians') who think we shall never understand the
relationship between mind and matter. It remains as Locke left it, a
rationally inexplicable matter-God's good pleasure. 'T'here are
even philosophers who think that some kind of Cartesian dualism
is true, and that the mind really is epiphenomenal-never causes
any physical events at all. They say this because they recognize that
the physical is a closed system. If there is a process that begins with
a pin being stuck in you and ends with a wince, then there is an entire physical chain from pin to wince that explains the wince. So,
they think, it has to be falsethat you wince because you are in pain.
This hit of common sense has to be given up. You wince because of
the physical pathways, not because of a