That ad hoc hypotheses are both repugnant and useful is a known fact. The joke of the biologist about teleology, that like a mistress, one wants to have it but not be seen with it, is more characteristic of ad hoc hypotheses. And, indeed, for mechanists, every teleological hypothesis is terribly ad hoc Spinoza called teleology the shelter of ignorance because it was ad hoc. When Newton said his gravity was not occult he argued from the fact that it was not ad hoc but a powerful explanation. Copernicus was indignant about Ptolemy's epicycles but had some himself, of course. The reduction of purpose to cause is ad hoc all too often. And Newton's optics was ad hoc, as William Whewell argued at great length. Now Copernicus' complaint is not in itself unreasonable: Ptolemy's epicycles were old hands and gained the legitimacy of regular customers, whereas his own were stop-gaps. This idea can be generalized.