 Some early theories of conditionals said that a counterfactual is true, or assertable, if and only if its antecedent, together with further “co-tenable” premises, implies its consequent (Ramsey 1931 commenting on Mill; Chisholm 1946; Goodman 1947, 1955; Mackie 1962). D. Lewis called such theories metalinguistic, the idea being that “if \(A\), \(C\)” is true provided there is an argument leading from the sentence \(A\) and additional premises to \(C\). On these accounts, the counterfactual is either a sentence meaning that some suitable argument backing it exists, or—as in Mackie’s version—it is itself an elliptical presentation of such an argument. The principal problem of any such theory is to specify which “additional premises” are suitable to be conjoined to a given antecedent and which are not. Goodman (1955) views the additional premises as general laws, supplemented by relevant factual conditions, but the problem is to determine which facts are to be assumed, and which facts are to be retracted. Rescher (1964) treats additional potential premises as ordered according to their position in a system of “modal categories”.