The international law on assassination is clear enough; assassination is murder and can be an act of aggression. The Dubai authorities are entitled to arrest and try the assassins of Mahmoud al-Mabhouh if they can catch them. They would be entitled to try to sentence them under Dubai law, though they would have a duty to provide a fair trial.

That is the ruling in law, but there is a separate issue of morality. Many Israelis undoubtedly feel that Mr Mabhouh was a dangerous terrorist and that Mossad — if it was responsible — was acting legitimately in self-defence. Yet there are interesting differences between different religions and different branches of Christian belief. The Catholic teaching looks for authority; the Protestant looks for justification.

The ethical question over the morality of killing terrorists seems to be the same as that raised in killing tyrants. Traditional Catholic teaching is to be found in the writings of the great medieval theologian St Thomas Aquinas. He considered it legitimate to kill a usurper, but only under a mandate from a legitimate authority. Murder requires an express mandate before a private person can lawfully kill even a tyrant. Otherwise killing a tyrant or a terrorist would be contrary to natural law.

This seems to be the ruling of the Council of Constance in 1415. Life, even of a terrorist, has an absolute value, and should be protected. The Protestant view is more utilitarian.

In the Reformation period, most of the leading Protestants were surprisingly strongly in favour of killing tyrants. The Scottish reformer, John Knox, affirmed that it was the duty of “the nobility, judges, rulers and people of England” to condemn Mary Queen of Scots to death. One leading German reformer, the “mild Melanchthon”, argued that the killing of a tyrant is the most agreeable offering a man can make to God.

The Catholic conscience would therefore ask the question about the killing of a terrorist: “What is the authority?” One could doubt whether Mossad had a valid mandate. The Protestant conscience should ask: “Does a state of war exist between the terrorist and his assassin?” That is the question that John Locke, in the late 17th century, asked about tyrants, and he was the intellectual founder of English Liberalism. I doubt if many Lib Dems nowadays have any sympathy with the morality of Mossad.

Flailing leader fluffs line

“A Future Fair For All” has been chosen by Gordon Brown as the Labour Party’s election slogan.

Only a politician with a tin ear would have made such a choice. Essentially, this sentence emphasises two consonants, which in conjunction are usually depressive. We all know what “F*** All” stands for. “A future of F all” is exactly what voters fear might be the outcome of a Labour victory. They do not need to be reminded. It is certainly not in Labour’s interest to remind them.

Countless words containing f and l spring to mind when one hears the Labour slogan. They are not flattering. In Michael Arlen’s classic bestseller of the 1920s, The Green Hat, the key sentence to describe Iris Storm, his heroine, is: “That lady was a fell lady.” Edgar Allan Poe wrote a macabre short story, The Fall of the House of Usher.

Phonetically, the word philistine resembles these words with f’s and l’s. So, in a title, does The Decline and Fall of the Roman Empire. Then there are words such as flatulent and fretful and fruitless and fallible. There is also Bentham’s sociological principle, which he called the “felicific calculus”. Flip, flap, flop and footling belong to the same group. All these associated words are downbeat. As Thomas Gray wrote: “The curfew tolls the knell of parting day” — that line has two f and four l’s and is the first line of the most famous expression of English melancholy of the 18th century.

Another phonetic link may be made to Pharsalia, the battle that decided the fate of Brutus and of the Roman Republic. “The fault, dear Brutus, is not in our stars, but in ourselves, that we are underlings.” With three l’s, this is a very liquid sentence.

The most penetrating f-l exchange also comes from Shakespeare, as one might expect. That is the brief and intense dialogue between the Scottish husband and his wife, Macbeth and Lady Macbeth.

He says: “If we should fail?” — and she replies: “We fail!” As indeed they did.

It was Mrs Siddons, the great actress, who changed the emphasis in that phrase. Before Mrs Siddons played Lady Macbeth, the words “We fail” were pronounced scornfully as though failure was unthinkable. She chose to pronounce them in a matter-of-fact way, on a falling tone:

“And if we fail, we fail.”

There is no doubt that failure is the most significant of the f/l words, the one Labour most has to fear, the foolish fellows.