I am not sure Fleetwood Mac were thinking of MI6 when they wrote Tell Me Lies way back in 1987. But the words came to mind while I was listening to the head of MI6 (Britain’s Secret Intelligence Service), Sir John Sawers today.

In the first ever public speech by an MI6 chief, he tried to explain something of the dilemmas faced by the intelligence services in fulfilling their commission on behalf of the people they defend. The main problem he faces in doing such a speech is to convince people that an organisation shrouded in secrecy and subterfuge can be trusted. Basically, how do we know when we are being lied to by those we pay to protect us and keep us ‘free’?

Blow the secrecy and you make most intelligence work – which is immensely complex and not usually for the fainthearted or the gullible – redundant. The very nature of the work means that games are played all the time – games in which reality, truth and honesty stand on a perilously thin line.

For example, should we be told about a threat to public order if the very telling (a) informs the protagonists that their ways and intentions are known – thus rendering the intelligence itself redundant and the process ended – , or (b) provokes the disorder it was intending to prevent?

Surely, not to tell the public everything is not the same as lying. The withholding of information is not the same as deliberately conning the public for sinister ends. If the public wants to be protected from (for example) Tube bombings, the public must trust the intelligence services to do its work appropriately. And this, as the MI6 boss said, pushes the service into very difficult areas of ethical propriety with which most of us do not have to engage.

The clear and unequivocal rejection of torture was important, but the statement itself begs further questions. The protected must also come to some conclusions about how high a price they are prepared to pay for their security: compelling the security services to refrain from action on certain intelligence even if that silence leads to a preventable terrorist attack being successfully executed?

This is the main problem. I remember meeting an old friend my GCHQ days in London. He was now working in counter-terrorism and observed his secrecy obligations to the letter in our conversation. But, he did express the frustration felt by his colleagues that the world knows (and has a view on) successful terrorist actions (such as the Tube bombings) whilst necessarily remaining ignorant of all the ‘attacks’ and plots that had been foiled, but could not be trumpeted. In other words, how does the public measure the success of the intelligence services when, by definition, it cannot know (a) what it does, (b) how it does it, and (c) where it is successful?

Yes, we have to be aware of the temptation in a secret world to get that world out of proportion and transgress on the wrong side of the ethical lines. However, we also need, at the very least, to recognise that the serious and sometimes unimaginable moral dilemmas faced by those we ask to protect us are tough, complex and hard to resolve tidily in a world that is hidden and mucky. The commentariat do their ethics sitting in a study; the intelligence services do theirs in a less comfortable environment. The commentariat usually has no idea of the ethical nightmares faced by those whom they easily and often ignorantly criticise. This doesn’t mean they shouldn’t question and scrutinise, but it does mean they should do so with some recognition of the limitations of their own experience and perspective.

Sir John Sawers was interesting today. But, try thinking about what he said and you realise that he actually said very little. That’s how it is. Maybe that’s how it has to be. As the widely divergent responses to last week’s Wikileaks disclosures demonstrated, this a grey area of moral life, not a black and white one.