Illustrations from the laboratory of the mind
Thought experiments are performed in the laboratory of the mind. Beyond that bit of metaphor it’s hard to say just what they are. We recognize them when we see them: they are visualizable; they involve mental manipulations; they are not the mere consequence of a theorybased calculation; they are often (but not always) impossible to implement as real experiments either because we lack the relevant technology or because they are simply impossible in principle, as when frictionless planes or a universe devoid of all matter are involved. If we are ever lucky enough to come up with a sharp defi nition of thought experiment, it is likely to be at the end of a long investigation. For now it is best to delimit our subject matter by simply giving examples; hence this chapter called ‘Illustrations’. And since the examples are so exquisitely wonderful, we should want to savour them anyway, whether we have a sharp defi nition of thought experiment or not.