then has the truth value meaning "Indeterminate" or "Inappropriate" when is false. In such a system, contraposition is not automatically allowed: "If then is not equivalent to "If then Consequently, "All ravens are black" is not equivalent to "All non-black things are non-ravens". In this system, when contraposition occurs, the modality of the conditional involved changes from the indicative ("If that piece of butter has been heated to 32 °C then it has melted") to the counterfactual ("If that piece of butter had been heated to 32 °C then it would have melted"). According to this argument, this removes the alleged equivalence that is necessary to conclude that yellow cows can inform us about ravens: In proper grammatical usage, a contrapositive argument ought not to be stated entirely in the indicative. Thus: From the fact that if this match is scratched it will light, it follows that if it does not light it was not scratched. is awkward. We should say: From the fact that if this match is scratched it will light, it follows that if it were not to light it would not have been scratched. ... One might wonder what effect this interpretation of the Law of Contraposition has on Hempel's paradox of confirmation. "If is a raven then is black" is equivalent to "If were not black then would not be a raven". Therefore whatever confirms the latter should also, by the Equivalence Condition, confirm the former. True, but yellow cows still cannot figure into the confirmation of "All ravens are black" because, in science, confirmation is accomplished by prediction, and predictions are properly stated in the indicative mood. It is senseless to ask what confirms a counterfactual. Differing results of accepting the hypotheses Several commentators have observed that the propositions "All ravens are black" and "All non-black things are non-ravens" suggest different procedures for testing the hypotheses. E.g. Good writes: As propositions the two statements are logically equivalent. But they have a different psychological effect on the experimenter. If he is asked to test whether all ravens are black he will look for a raven and then decide whether it is black. But if he is asked to test whether all non-black things are non-ravens he may look for a non-black object and then decide whether it is a raven. More recently, it has been suggested that "All ravens are black" and "All non-black things