Think: A Compelling Introduction to Philosophy by Simon Blackburn

There is no independent variation. There can't he Zombie or Mutant gases, in which the kinetic
energy of the molecules either issues in no temperature at all, or issues in different temperatures from those associated with the same
energy in other gases.
    On the other hand it is not simply reason or thought or mathematics that enabled scientists to equate temperature with mean kinetic energy. The breakthrough was not a priori, armchair analysis
of what is meant by temperature, but took experiment and observation, and general theoretical considerations. The result was not
purely a priori, but at least mostly a posteriori. The relation is not
one that could be worked out in advance just by mathematics or by `clear and distinct ideas, like the fact that a circle on a tilted plane
casts an ellipse.

    In general, in science, when one theoretical term or property,
like temperature, becomes identified with another (here mean kinetic energy of constituent molecules), the link is given by bridge
principles that are part of the theories of the sciences in question.
So, for example, the current identification of genes with hits of
1)NA happens because in classical biology genes are defined in
terms of their function in making characteristics heritable, and
now in molecular biology it turns out that bits of DNA are the
things that have that function. Notice that analysis is not entirely
absent. We have to know what genes are meant to do before the
equation can be made. But the big discovery is the contingent, scientific discovery of what it is that does what they are defined as
    If we modelled our approach to the mind-brain problem on scientific reductions of the kind just described, we would find some
physical state characteristic of people sharing some mental state.
So, for instance, we might find that all and only people in pain
share some brain state (often indicated vaguely by saying that their
`C-fibres are firing'). And then it would be proposed that this then
is the state of being in pain, just as some bits of I )NA are genes.
Once again, there would be a complete reduction of the mental to
the physical.
    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