BOOK DISCUSSION: Roy SORENSEN'S THOUGHT EXPERIMENTS 

1. Precis of Thought Experiments 

Roy A. SORENSEN New York University 

Imagination is associated with escapism, free association, and unfettered thought. 
Yet thought experiments intertwine with all sorts of reasoning and even in the 
appraisal of argument. Typically, the validity of an argument is first tested by 
trying to imagine a situation in which the premises are true and the conclusion 
false. For example, George Simmel invalidated Nietzsche's argument for the 
eternal return by envisaging a universe containing three wheels on a single axle 
spinning endlessly at rates n, 2n, and pi/no (The incommensurability introduced 
by the irrational number pi ensures that the wheels never re-align.) Enthymemes 
are reconstructed with a curious crew of fictions: the devil's advocate, the 
reasonable man, the ideal thinker, etc. Definitions are refuted by (often highly) 
hypothetical counterexamples: Russell's five minute hypothesis, the Ship of 
Theseus, Aquinas' cannibals. Imagination also figures prominently in less 
understood acts of clarification such as idealization. 

True, logicians often try to break free of any reliance on imagination. The 
formal strategy is to disinterpret an argument into inert symbols and let the proof 
proceed by purely syntactic means. But this two dimensional symbol 
manipulation only applies to arguments that have been translated into the 
calculus. Thought experiment is well-represented at this interface between 
informal and formal logic because the translator must test the faithfulness of the 
translation by trying to imagine whether the statement could be true while its 
translation is false. Crucial logical points have themselves been made by thought 
experiment. Recall Lewis Carroll's debate between Achilles and the Tortoise, 
Quine's radical translator, and Wittgenstein's woodcutters. Hypotheticals have 
enriched our logical vocabulary with 'grue', 'tonk', and 'quus'. 

Nevertheless, methodical sorts who demand clear reasoning from clear 
sources of evidence are apt to be suspicious. Thought experiments seem to give 
you something for nothing. The thought experimenter starts out ignorant. Then 
he sits in his armchair. Instead of looking, smelling, listening, and so forth, he 
blocks out empirical data and uses his imagination. He then rises with the answer. 
What, short of parapsychology, could account for this cognitive transformation? 

The skeptic challenges the presupposition that the thought experiment 
proves something. He grants that the thought experiment might be relevant to the 
context of discovery (like a dream) but not the context of justification. My main 
response in Thought Experiments is to present a variety of scientific thought 
experiments that are acknowledged as proof by scientists. I then rely on meta-



386 Roy A. Sorensen 

philosophical gradualism (the view that science and philosophy differ in degree, 
not kind) to extend lessons about scientific thought experiments to philosophical 
ones. 

Some of the scientific thought experiments are conceptual but others deliver 
empirical answers. Hence these cases show that we cannot explain their success 
as just the deduction of tautologies or as an appeal to ordinary language. 

Einstein's mentor, Ernst Mach, suggested that the empirical reliability of 
thought experiment is due to the generate and eliminate mechanism of natural 
selection. Proto-human beings who had a more accurate feel for what could not 
happen ad an edge on predicting and controlling nature and each other. This sense 
f the absurd has been refined into a touchstone of truth. For we can eliminate an 
hypothesis by showing that it has an absurd consequence. 

I defend Mach's evolutionary strategy as basically sound. I update Mach's 
biology and put his epistemology in a reliabilist framework. A fully general 
account, one that can handle all thought experiments from Aesthetics to Zoology, 
must explain how non-perceptual processes enhance epistemic authority. 

There are a number of models of armchair inquiry that seem sensible and 
enlightening: recollection, transformation, homuncular, rearrangement, and one I 
call the cleansing model. Psychology is too young to allow much development of 
the first four. However, a special case of the cleansing model lets me lean on 
standard logic and offers a way of domesticating some of Thomas Kuhn's wild 
insights about thought experiments. In particular, I argue that each thought 
experiment can be regimented so that it fits one of two schemas. One is based on 
the refutation of necessities and the other targets possibilities. 

Under this approach, thought experiments are devices to expose and 
eliminate inconsistency. Each thought experiment can be represented as a stylized 
paradox, that is, a small set of individually plausible but jointly inconsistent 
propositions. The official aim of the thought experiment is to refute the source 
statement but the thought experimenter can pursue a richer agenda through 
indirect means just as conversationalists can perform lots of trick bounces off of 
the official Gricean maxims of conversation. So although my model is artificially 
rigid, it is intended to support highly flexible uses of thought experiment. 

The paradox model is also intended to work for regular experiments. One of 
the themes of the book is that thought experiment is (a limiting case of) 
experiment. I argue that thought experiment evolved from experiment by an 
attenuation of the execution element and a consequent elaboration of its other 
elements. Regular experiments have the same modes of enlightenment (though in 
lesser degree) as thought experiments (organizing data, making subtle 
connections, grounding hypotheses into tests) plus the obvious mode of 
enlightenment: fresh information through the execution of the experiment. So 
lessons learned about thought experiment enrich our understanding of ordinary 
experiments just as the recent growth in understanding of ordinary experiments 
helps us better understand thought experiments. These lessons are applied in the 
final chapter centering on fallacies ill' thought experiment. I conclude that 
although thought experiments are susceptible to various sorts of misuse and 



1. Precis a/Thought Experiments 387 

abuse, they are generally reliable when used as part of a wide portfolio of 
techniques. 

ROY SORENSEN 
DEPARTMENTOFPHftOSOPHY 

NEW YORK UNIVERSITY 
100 WASHINGTON SQUARE EAST 

NEW YORK. NY 10003-6688 
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