popper-logic-scientific-discovery

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new appendices
a gravitational field may proceed on curved paths. This use is important
and legitimate.
The main purpose of this note is to issue a warning against what
may be called the apologetic use of imaginary experiments. This use goes back, I
think, to the discussion of the behaviour of measuring rods and clocks
from the point of view of special relativity. At first these experiments
were used in an illustrative and expository way—a perfectly legitimate
usage. But later, and in the discussion of quantum theory, they were
also used, at times, as arguments, both in a critical and in a defensive or
apologetic mood. (In this development, an important part was played
by Heisenberg’s imaginary microscope through which one could
observe electrons; see points 9 and 10 below.)
Now the use of imaginary experiments in critical argumentation is,
undoubtedly, legitimate: it amounts to an attempt to show that certain
possibilities were overlooked by the author of a theory. Clearly, it must
also be legitimate to counter such critical objections, for example, by
showing that the proposed imaginary experiment is in principle
impossible, and that here, at least no possibility was overlooked. 4 An
imaginary experiment designed in a critical spirit—designed in order
to criticize a theory by showing that certain possibilities have been
overlooked—is usually permissible, but great care must be taken with
the reply: in a reconstruction of the controversial experiment, undertaken
in defence of the theory, it is, more particularly, important not to
introduce any idealizations or other special assumptions unless they are
favourable to an opponent, or unless any opponent who uses the
imaginary experiment in question would have to accept them.
(2) More generally, I think that the argumentative use of imaginary
experiments is legitimate only if the views of the opponent in the
argument are stated with clarity, and if the rule is adhered to that the
idealizations made must be concessions to the opponent, or at least acceptable to
the opponent. For example, in the case of Carnot’s cycle all idealizations
introduced increase the efficiency of the machine, so that the opponent
to the theory—who asserts that a heat machine can produce
4 For example, my own experiment of section 77 has been shown to be in principle
impossible (from the quantum-theoretical point of view) by Einstein in his letter printed
in appendix *xii; see the note on p. 232 and notes *3 and *4 to section 77.