A defendant who had been condemned in an action was under an obligation to satisfy the
judgment—i. e. to pay to the plaintiff the sum of money which the iudex had awarded;
and the actio iudicati was the mode which the successful party had to adopt in order to compel
the defendant to fulfil his obligation. The ground of the actio iudicati being the judgment
debt, the defendant was not allowed to go behind the judgment and dispute the original claim,
but might set up the plea that the judgment was invalid. If defeated in the action he was
mulcted in double the original amount.