“The particular type of estoppel relied upon by the husband is estoppel per rem judicatam. This is a generic term which in modern law includes two species. The first species, which I will call “cause of action estoppel,” is that which prevents a party to an action from asserting or denying, as against the other party, the existence of a particular cause of action, the non-existence or existence of which has been determined by a court of competent jurisdiction in previous litigation between the same parties. If the cause of action was determined to exist, i.e., judgment was given upon it, it is said to be merged in the judgment, or, for those who prefer Latin, transit in rem judicatam. If it was determined not to exist, the unsuccessful plaintiff can no longer assert that it does; he is estopped per rem judicatam. This is simply an application of the rule of public policy expressed in the Latin maxim “Nemo debet bis vexari pro una et eadem causa.”
“There is one observation which I wish to make upon the passage from the judgment of Diplock L.J. which I have just quoted. This is that the principle of merger to which Diplock L.J. refers as applying where the cause of action was determined to exist, in the sense that judgment was given upon it, cannot be described simply as a species of estoppel. The principle, which is sometimes called the doctrine of merger in judgment, is that a person “in whose favour an English judicial tribunal of competent jurisdiction has pronounced a final judgment . . . is precluded from afterwards recovering before any English tribunal a second judgment for the same civil relief in the same cause of action:” see Spencer Bower and Turner, Res Judicata, 2nd ed. (1969), pp. 355 et seq. The basis of the principle is that the cause of action, having become merged in the judgment, ceases to exist, as is expressed in the Latin maxim transit in rem judicatam: see King v. Hoare (1844) 13 M. & W. 494, 504, per Parke B., cited by Lord Penzance and Lord Blackburn in Kendall v. Hamilton (1879) 4 App. Cas. 504, 526 and 542.”