And the latest leak against Julia Gillard is a leak of the most smearing and unpluggable kind. It's not a leak of a cabinet submission, which could be debated on issues of fact.

It's a leak about what she is alleged to have argued inside cabinet, designed to make her appear opportunistic and uncaring.

And because it's a version of a conversion - a very selective version of a conversation according to cabinet sources - it is impossible to refute.

Cabinet confidentiality is supposed to be one of the immutable laws of our system of government - so that ministers can freely and frankly discuss policy inside the cabinet room.

Once they step outside the room, all freeness and frankness is supposed to cease and any divergence from the collective decision is supposed to be punishable by immediate resignation (which is why those attacking Penny Wong for refusing to contradict the government position on gay marriage presumably think it's better for her to be forthright about her personal views from the backbench).

Of course, like most immutable laws, cabinet confidentiality turns out to be mutable in some instances - usually when ministers want information in the public domain - like when the Special Minister of State, Joe Ludwig, released cabinet documents in May showing when the government had sought an exemption from the normal rules for advertisements about the mining tax.

And as with a law of any description it has often been broken - very often in Labor's last term of office under prime ministers Hawke and Keating.

Bob Hawke was so incensed about the leak of three entire cabinet submissions on communications policy he wrote a stern letter to ministers about security measures.

It, too, was leaked. Nor was the Howard government leak free, especially in its first term when several submissions on possible budget savings were leaked. Ditto the Rudd government, although taking most decisions in a group of only four ministers narrowed the possibilities.