One treatise from the nineteenth century put it this way: "The interest of the defender to see that the pursuer has a proper title is obvious; for, otherwise, the discharge granted on payment of the claim may be insufficient, and a decree of absolvitor will not afford a plea of res judicata." (123) Kames explained that title and interest were required because a "decree is effectual between the litigants only; and a court will give no countenance to an action or to a defence that cannot be effectual." (124) Both writers were saying essentially the same thing.
(187) They argued that it was a maxim of Scottish law that "no person is bound to answer as a defender in any case, or with any pursuer where an absolvitor will not afford him an exceptio res judicata against a similar process, raised at the instance of any other person." (188) The Court of Session overruled this objection, saying that the objection did not apply regarding damages and a declarator to prevent repetition of the damages.
Defendants too had to have title and interest, although "as every defendant finds in his account in an absolvitor, it is obviously his interest to defend himself against a claim that is not founded on law." Id.
[If such were permitted, the absolvitor in the popular action] will not operate [for the defender as] an exceptio rei judicate.