Hypocrisy is like dry rot. Air and light will kill her, too. Her enemy, too, is the sun and wind. Her friends are anxiety, discretion, envy, embarrassment and fear.

She too loves humans and their abodes, full of dark corners, painted surfaces and secrets starved of oxygen. Hypocrisy too becomes a kind of monster in our mental attic. From there she seeks out the paintwork. She takes on a life of her own, spreading her tentacles in whispers through society, demanding discretion as she goes, then slithering beneath the silences so that knowledge becomes guilt, and any polite veneer — the realm of what is known but not said — implicates the bystander who knew but did not say. Good manners become hypocrisy’s friend.

And in modern Britain hypocrisy finds no darker or more airless attic than that in which, as a society, we keep our involvement with drugs.

Our problem is not the drugs but the hypocrisy. I know nothing of David Cameron’s youth, his family, his past or indeed his present. If like me and so many of my friends and colleagues he has ever smoked cannabis or tried cocaine, that would be of little consequence to me, and no surprise at all. That he has a close relative who has had difficulties with drugs, then he is one of millions. It may be that to have been touched by these experiences gives him understanding. They are part of the experience of many, perhaps most, in the generation of which Mr Cameron is part. They make him normal.

Yet this kind of normality cohabits with a capacity, when published, to shock. Such cohabitation is a very strange thing. How can it be that facts about an individual can shock people whom it does not surprise?

My failure to comprehend this most curious of cohabitations landed me in all kinds of trouble once when I mentioned on television that Peter Mandelson was gay. I overlooked the difference between knowledge and publication (or broadcasting). The step from common knowledge to published fact, though a short distance, may span a chasm into which a whole career could tumble. It is that chasm on whose brink Mr Cameron now hesitates before the final step.

He should take it. We British have got ourselves into an unholy tangle about truth, secrecy, and drugs. Silence can be toxic, and hypocrisy’s friend.

Some drugs are very harmful and their use should be discouraged, perhaps forbidden. But most of us have done things that are harmful and forbidden. I have driven cars much too fast, endangering life. I have driven cars when under the influence of alcohol. I have deliberately caused a bomb scare at an IRA fundraising meeting in Connecticut. I have failed to volunteer payment for rail journeys when nobody checked tickets. I have made payments in cash to people wishing to avoid tax, and benefited from the discount. I have put my signature in loco parentis on an application form for a parachute jump for a young man whose parent I undoubtedly was not, and who turned out (unknown to me) to be the son of the proprietor of this newspaper. I have no difficulty with any of these admissions.

The point about such crimes is not that one can boast about them — I am ashamed of many — but that one can tell the truth without any great fear that the disclosure will destroy career or reputation.

Drugs are different. Nobody knows quite why, when or how we as a society decided that they should be special, but some kind of a collective cultural decision does seem to have been taken — and relatively recently, certainly not before the 1930s. We appear to have placed drugs on a small list of commonplace truths which must not be published.

Alcohol abuse, incredibly disruptive as it may be, is not on that list, though it almost destroyed the lives of people such as George W. Bush and Alastair Campbell. It is tempting to conclude that an honorary exemption has been made for alcohol because drinking is both commonplace and legal.

But so is sexual indiscretion. Yet sexual indiscretion is on the list. Though tens of millions of us are guilty and it isn’t against the law, sexual indiscretion retains — where published — a potency to damage or even destroy a career, especially in public life. Why? Because it is secret. Because it is covered by silence. And beneath that silence, like dry rot beneath the paintwork, hypocrisy spreads her tentacles.

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