are reduced to statistical mechanics. Boyle’s Law states this: PV = kT. So if
the volume (V) of a gas is kept constant, an increase in temperature (T) will
cause a corresponding increase in pressure (P). This law can in fact be
derived from statistical mechanics, together with the ‘bridge principles’
that pressure is force per unit area, and temperature is mean molecular
momentum. For as the average momentum of the molecules (the tem-
perature) is increased, so the force per unit area exerted on the surface of
the container (the pressure) will also increase, if that surface area remains
constant.
Just as there have been very few successful analytic reductions, so, too,
there exist few successful inter-theoretic reductions. The reason, in this
case, lies with the phenomenon ofmultiple realisability. It appears to be
quite common for laws in the special sciences (chemistry, biology, neurol-
ogy, psychology, and so on) to be multiply realised in lower-level mechan-
isms. If there are a variety of diVerent physical mechanisms, involving a
variety of diVerent physical properties Pi, any one of which is suYcient to
realise a property T in a special-science law, then it will not be possible to
identifythe special-science property T with any single physical property.
This sort of situation is especially likely to arise in the case of biology and
psychology, where we know that evolution can come up with a number of
diVerent mechanisms to perform the same function. In which case we
should not expect to be able toWnd reductive accounts of psychological
properties, including semantic properties. So, if realism requires naturalis-
ation, it becomes crucial to know whether naturalisation, in turn, must
require reduction.
5.2 Naturalisation without reductionPlainly, belief in the reality of some property cannot,in general, require a
successful reduction of that property into other terms, on pain of vicious
regress. Nor is it very plausible to maintain that all propertiesabove the
level of basic physicsneed to be reduced to the terms of that physics in order
to be shown to be real. For then we may have to deny the reality of
chemical and biological kinds, as well as the reality of psychological ones.
For there are very few successful inter-theoretic reductions to be found at
all, let alone reductions to the vocabulary of basic physics. Rather, we
should accept that the existence of a variety of special sciences is a per-
manent, irreducible, part of our world-view, reXecting the way in which the
natural world is organised in terms of laws and principles operating at
diVerent levels of generality. And then all we need do in order tonaturalise
some property, is show that itWgures in the laws of some or other special
science, in whose persistence we have good reason to believe.
186 Content naturalised