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Is it better to say argumentum a/ab contrario or e/ex contrario?
It seems that both are acceptable but in most Romance languages it is a contrario.
The movement out/from is not clear/explicit/graphic and it is a concept/mental abstraction that "moves", not a physical object.
EDIT secondary questions following up to clarify as suggested Why ex and not ab? Why do most Romance Languages have a contrario instead? How did ex contrario evolve to a contrario?
This is just a follow-up post to Sebastian's answer, which is correct for Classical Latin. It could be useful to add that the expression a contrario (often used as part of argumentum a contrario) comes from scholastic Latin. The expression a contrario is typically used in Romance-speaking countries, in English-speaking countries, in Slavic-speaking countries, etc. In contrast, the classical form is more typically found in German-speaking countries, in Scandinavian countries, etc. As for OP's question ("How did ex contrario evolve to a contrario?"), I think it is not a matter of "evolution" (?) but rather of "replacement" of e(x) 'out of, from' by a(b) 'from' in this expression. As for the question why such a substitution did not (typically) take place in, let's say, Sebastian's or Joonas's countries (Germany and Finland, respectively; cf. supra), I don't know.
