Long before the arrest of Damian Green, senior Conservatives were given legal advice about the difference between receiving leaked documents and inducing civil servants to leak them. This suggests that they were well aware of the danger - political as well as legal - of either placing (or "grooming", in the phrase used by the police) political spies within the Whitehall machine.
This goes some way to explaining the answer to that oft-asked question: why was a senior Tory politician arrested and held by police for nine hours when other opposition politicians, like the young Gordon Brown were not - even when they boasted of the leaks they received?
The police who cross-examined Mr Green, the Conservatives' immigration spokesman, suggested that he had not simply received leaked documents but had, in their controversial phrase, "groomed" the civil servant who allegedly leaked them - a man who had been a Tory activist and who applied for jobs in Mr Green's office. The Home Office called in the police after the leaking of 20 politically sensitive documents.
The Home Secretary has refused to apologise for the police's actions. Indeed, Jacqui Smith has told colleagues that she doesn't believe that the police did anything wrong.
However, the leader of the Commons, Harriet Harman, has made clear her determination to investigate both the law under which Mr Green was arrested and the processes which led the Commons authorities to allow the police to search his Commons office.
What explains the difference? Well, 25 years ago, Ms Harman - then a civil liberties lawyer and young opposition MP - was taken to court by the Home Office for - you guessed it - leaking court documents.
This is adapted from the script of my piece on this morning's Today programme.