popper-logic-scientific-discovery

462
new appendices
the phenomenalist modal translation of ‘At the place x is a flower-pot’
will be true for all those places x which, for some physical reason or
other, nobody can look at. (Thus there is a flower-pot—or whatever else
you like—in the centre of the sun.) But this is absurd.
For this reason, and for many other reasons, I do not think that there
is any chance of rescuing phenomenalism by this method.
As to the doctrine of operationalism—which demands that scientific
terms, such as length, or solubility, should be defined in terms of the
appropriate experimental procedure—it can be shown quite easily that
all so-called operational definitions will be circular. I may show this
briefly in the case of ‘soluble’. 27
The experiments by which we test whether a substance such as
sugar is soluble in water involve such tests as the recovery of dissolved
sugar from the solution (say, by evaporation of the water; cf. point 3
above). Clearly, it is necessary to identify the recovered substance, that
is to say, to find out whether it has the same properties as sugar. Among
these properties, solubility in water is one. Thus in order to define ‘x is
soluble in water’ by the standard operational test, we should at least
have to say something like this:
‘x is soluble in water if and only if (a) when x is put into water then it
(necessarily) disappears, and (b) when after the water evaporates, a
substance is (necessarily) recovered which, again, is soluble in water.’
The fundamental reason for the circularity of this kind of definition
is very simple: experiments are never conclusive; and they must, in
their turn, be testable by further experiments.
Operationalists seem to have believed that once the problem of subjunctive
conditionals was solved (so that the vacuous satisfaction of the
defining conditional could be avoided) there would be no further obstacle
in the way of operational definitions of dispositional terms. It
seems that the great interest shown in the so-called problem of subjunctive
(or counter-factual) conditionals was mainly due to this belief.
27 The argument is contained in a paper which I contributed in January 1955 to the
Carnap volume of the Library of Living Philosophers, ed. by P. A. Schilpp. It is now in my
Conjectures and Refutations, 1965, ch. II, p. 278. As to the circularity of the operational
definition of length, this may be seen from the following two facts: (a) the operational
definition of length involves temperature corrections, and (b) the (usual) operational definition
of temperature involves measurements of length.