  Let’s clarify the sense of condition (F) with an example. Consider a language LAr+ which is like LAr but has besides another individual constant, “2”, and another dyadic predicate, “Pd”, whose desired interpretations are the number 2 and the relation of being the immediate predecessor of, respectively. Let K be the following set of sentences of LAr+: {“∀x(Nx→¬Mx0)”, “N0”} (these sentences are true); and let X be the sentence “¬M00”. The argument with premises K and conclusion X is intuitively logically correct; thus, according to Tarski, it must verify condition (F). This means that any argument obtained from it by uniform replacement of non-logical constants by non-logical constants must be an argument where it is not the case that the premises are true and the conclusion false. Let’s suppose that the non-logical constants of LAr+ are “0”, “N”, “M”, “2”, and “Pd”. Replace “0” with “2” and “M” with “Pd” in the argument with premises K and conclusion X and call the resulting set of premises and conclusion “K′” and “X′”. That is, K′ is {“∀x(Nx→¬Pdx2)”, “N2”} and X′ is “¬Pd22”. In virtue of condition (F), the argument with premises K′ and conclusion X′ must be an argument where it is not the case that the premises are true and the conclusion false; and that’s in fact the case: the first premise is false, and the second premise and the conclusion are true.