A counterfactual conditional relation is a conditional relation in which the form of expression of the antecedent and consequent marks them as imagined, nonfactual states or events.
Comrie 1986: 89-90 establishes that the putative English counterfactuals do not contain the nonfactuality of either the antecedent or the consequent as part of their inherent meaning. Thus, If you gave me a kiss, I’d buy you a beer does not express the impossibility of either the kiss or the beer. Additionally, in If the butler had done it, we would have found just the clues that we did in fact find, it is clear that the consequent is factual, and factuality of the antecedent is possible.
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Longacre, Robert E. 1985. "Sentences as combinations of clauses." In Shopen 1985c
Longacre, Robert E. 1983.The grammar of discourse. New York: Plenum.
Crystal, David. 1985.A dictionary of linguistics and phonetics. 2nd edition. New York: Basil Blackwell.
Comrie, Bernard. 1986. "Conditionals: A typology." In Traugott 1986
Beekman, John, John Callow, and Michael Kopesec. 1981.The semantic structure of written communication. Dallas, TX: Summer Institute of Linguistics.