Latin for: “(argument) on the matter”. Denotes a factual argument – in contrast to an ad hominem argument, which is directed in an unobjective manner against the person making an argument.
The term res refers solely to the fact that an argument addresses the “matter” being discussed, and serves to distinguish it from ad hominem arguments, which target the person rather than the argument.
Even such arguments “on the matter”, however, can still be fallacious or unfair in other ways. Alone from an argument being ad res does not follow that it is also valid.