I believe that you are referring to the case of Wilkinson v. Downton, in which Mr. Justice Wright presided. I have changed your question to reflect that.
The rule that was established by this case has to do with what acts by a defendant make them liable for a tort. The rule says that any act which is calculated to cause harm and does cause harm can be actionable, even if the harm is merely psychological. This is why Downton was liable for the mental distress caused to Mrs. Wilkinson by the lie that he told her.
So, the rule in this case has to do with the intentional infliction of emotional pain. Wright held, for the first time, that this was a tort similar to a physical assault.