The impulse for change comes from two sources. One is the realisation by the security services that, if they are to retain public support and funding, they must explain why they are still necessary after the ending of the Cold War. Ms Rimington made a strong and lucid case for her department by describing the shift of resources from counter-espionage (although Russian espionage is increasing) to counter-terrorism, in response to both Irish and international terrorists, particularly those trying to gain access to nuclear weapons.
If she managed to justify her own existence, she may also have gone some way towards meeting the other reason for her public appearance: the public has grown more distrustful of authority in general and of governments in particular, a fact that is reflected in less deferential media.
This is on the whole a healthy change, since it should sharpen the checks and balances that are needed to control secret services in a democracy. As Ms Rimington admitted in some passages of her speech, while seeming to deny it in others, there is an inevitable tension between liberty and security, as between democracy and secrecy. The only way to manage that tension is to ensure that the secret services are accountable to representative bodies without jeopardising the secrecy of their operations. The balance will always be delicate.
During the Cold War the distinction between legitimate political dissent and subversion became blurred because paid agents of the Soviet Union were at work in the British Communist Party, the trade unions and the peace movements. Surveillance of their activities inevitably embraced people who regarded themselves as engaged in nothing more threatening than legitimate political dissent.
The struggle against the IRA should be morally simpler because it enjoys even less public support than Communism. Even so, the danger that secret activities may become corrupted by the means they employ remains a constant threat to civil liberties. Ms Rimington has given us useful reassurances. It is now up to Parliament to reinforce them.