by Scott Aikin and Robert Talisse
Argument by analogy is like a powerful chainsaw. If you know how to use it, you can do some nice work. But if you aren't careful, you can make a big mess, and maybe hurt yourself as well.
The core form of argument by analogy is to infer from two things' acknowledged similarities that those things have further, as yet unacknowledged, similarities. One begins with a generally familiar phenomenon (the proximal object of the analogy), and then attributes salient features of the proximal object to another, less familiar matter (the distal object of the analogy). One the basis of the analogy, it is established that what is true of the proximal object is also true of the distal object.
Argument by analogy is a particularly widely used tool throughout Philosophy. Plato's analysis in The Republic of the good man runs on an analogy between justice in the soul and justice in the city. Plato's argument is that, as justice in the city consists of a certain kind of hierarchical order, the just man's soul manifests the same structure. Judith Jarvis Thomson's famous violinist case analogizes unplanned pregnancies to being kidnapped, taken to a hospital, and hooked up to a famous violinist who needs use of your organs in order to stay alive. And Paley's argument from design starts with the hypothesis that the world's functions (and those of many bodies within the world) are like those of a watch, whose very existence suffices to demonstrate the existence of a watchmaker.
It is not difficult to see how arguments by analogy may be challenged. These arguments depend on there being good reason for accepting the proposed analogy in the first place. Accordingly, many of those who reject the conclusions of Plato's, Thomson's, and Paley's arguments contend that the analogies themselves are at least as controversial as the conclusions they are supposed to support. Consider Paley's watchmaker argument. The premise that the world is analogous to an artefact goes a long way towards reaching the conclusion. If you thought the conclusion unacceptable, then the analogy just won't look right.
Hence arguments by analogy face a particular dialectical obstacle. Only those already roughly in agreement with the desired conclusion will readily grant the initial analogy. And so the analogies are either unnecessary or they are proxies for arguments that establish the appropriateness of the analogies. Arguments by analogy, then, are either otiose or insufficient.