WHEN I studied philosophy in Liverpool in the olden days, a particular phrase was drummed into us: “Ought implies can.” It means no one has a duty to do something which he lacks the capacity to do. It also means: “Don’t promise something you can’t carry out.”

I thought of that when I heard David Cameron’s announcement that British jihadists who travel abroad to fight will be prevented from returning. Dave said the special exclusion orders - which could last for two years or more - would bar suspected fighters from entering the UK unless they agreed to strict controls. Officials would also have the power to stop would-be jihadists leaving the UK.

But would they? Promises, promises.

Just how does the Government imagine that it can prevent anyone from entering this country? We know from the official figures that 100,000 more come into Britain each year than the numbers who leave. That’s a new medium sized town every 12 months. And that’s only the number of legal immigrants.

Shockingly, the authorities admit that they simply haven’t the foggiest idea of how many enter illegally. This is not a Ukip scare story or something construed by nasty parties such as the BNP.

A few weeks ago the BBC’s File on Four confirmed in precise detail that our borders are, as the presenter put it, “porous”. When even the BBC wakes up to the fact that we have no control over the number of illegal immigrants, then we know something is amiss.

File on Four hazarded a guess and suggested the figure might run into thousands each month. The programme told us how the illegals are managing to get in – often risking death in the process.

Some are hiding themselves in car boots in the French ports and hoping to cross the channel as freeloaders on returning British holiday-makers or businessmen. Or they stow away on lorries.

Most dangerously of all, they get themselves into the massive cargo containers and come in among our imports. Then there are those who land in small boats on the sparsely-populated parts of our coast.

And, because our coastline stretches to thousands of miles, they don’t lack for opportunity. Now Dave is promising that in future he will keep tabs on those returning. But, since he hasn’t managed to do this in the past, how can we trust him to do it in the future?

And then we are told that returning jihadists must comply with “strict controls”. As if terrorists could be relied upon for their compliance. Illegals are not noted for their obedience to the law, are they?

Even supposing strict controls were enforceable, who would administer these controls? The overstretched police or the customs officers, the coastguards, the magistrates, dad’s army? Or does the Government intend to recruit a new and secret police force to do the job? And in any case, who is to pay for all this supervision? Well, we all know the answer to that one.

Now, Dave made that announcement about the ban on returning jihadists while he was in Australia.

I have a practical suggestion: in the interests of our nation as a whole, can’t we contrive to slap a ban on Mr Cameron to prevent his return? It happens that at the moment the Australian people are fortunate to have a real Tory in charge in the person of their splendid Prime Minister Tony Abbot. Couldn’t Mr Abbot be persuaded to put a restraining order on Dave and confine him to the antipodes – like we used to do with the convicts?