¹ See for example Hempel, C. G., Aspects of Scientific Explanation (New York : The Free Press, 1966) p. 339Google Scholar. Note that in section 6 of Chapter IV of Fact, Fiction, and Forecast Goodman describes some exceptions to his principle. But they are of a different sort to those that I shall discuss.

² This formulation is essentially due to R. C. Stalnaker, in ‘A Theory of Conditionals’, Studies in Logical Theory (A. P. Q,. supplementary monograph series, 1968). A persuasive and rigorous sharpening of it will appear in David Lewis' forthcoming book Counterfactuals (Blackwell, probably 1973).