There is a whole pile of reasons why Islamists want to stop her explanations from being aired. But why -- when the attacks keep on happening -- do our own societies collude with such sinister people to keep ourselves the dark?

That this type of campaign can succeed -- that speakers can be stopped from speaking in Western democracies because of the implicit or explicit threat of violence -- is a problem our societies need to face.

Like other criticisms of Hirsi Ali, the effort was to portray her as the problem itself rather than as the response to a problem.

Only a fortnight after a vehicular terrorist attack in Westminster, London, another similar attack took place in Stockholm, Sweden. On one of the city's main shopping streets, a vehicle was once again used as a battering-ram against the bodies of members of the public. As in Nice, France. As in Berlin. As so many times in Israel.

Amid this regular news there is an air of defeatism -- a terrible lack of policy and lack of solutions. How can governments stop people driving trucks into pedestrians? Is it something we must simply get used to, as France's former Prime Minister Manuel Valls and London's Mayor Sadiq Khan have both suggested? Must we come to recognise acts of terror as something like the weather? Or is there anything we can do to limit, if not stop, them? If so, where would we start? One place would be to have a frank public discussion about these matters. Yet, even that is easier said than done.

There is a terrible symmetry to this past week in the West. The week began with the news that the Somali-born author and human-rights activist Ayaan Hirsi Ali had been forced to cancel a speaking tour in Australia. "Security concerns" were among the given reasons. A notable aspect of this issue, which has been made public, is that one of the venues at which Hirsi Ali was due to speak was contacted last month by something calling itself "'The Council for the Prevention of Islamophobia Incorporated". Nobody appears to know where this "incorporated" organisation comes from, but its purported founder -- Syed Murtaza Hussain -- claimed that the group would bring 5000 protestors to the hall at which Hirsi Ali was scheduled to talk. This threat is reminiscent of the occasion in 2009 when the British peer, Lord Ahmed, threatened to mobilise 10,000 British Muslims to protest at the Houses of Parliament in Westminster if the Dutch politician Geert Wilders were allowed to speak. On that occasion -- as on this one -- the event was cancelled. Promises to mobilise thousands of angry Muslims can have such an effect. But the long-term implications often get lost in the short-term outrage.

Other attacks on Hirsi Ali began, in fact, weeks before her now-cancelled tour had been due to start. On the web, for instance, a widely-watched video was disseminated showing a group of headscarf-covered Australian Muslim women. All were attacking Hirsi Ali and protesting her appearance in the country. Addressing her directly, they complained that, "Your narrative doesn't support our struggles. It erases them."

Like other criticisms of Hirsi Ali, the effort was to portray her as the problem itself rather than the response to a problem. Once again, mixing up (deliberately or otherwise) the arsonist and the firefighter, such groups present a homogenous, agreed-upon opinion -- or "narrative" -- as the only necessary answer to any problems that may or may not exist. Hirsi Ali, according to them, thinks the "wrong" things and says the wrong things. Therefore she must be stopped.

That this type of campaign can succeed -- that speakers can be stopped from speaking in Western democracies because of the implicit or explicit threat of violence -- is a problem our societies need to face. But in the meantime, we also have to face the reality that a shut-down of opinion has on our public policy as well as our public discourse.

What, after all, is the acceptable discourse -- or "narrative" -- on which we can agree to speak about the attacks in Stockholm, Berlin, Nice and elsewhere? Can the discussion be allowed to include the Islamic portion? Can anyone be allowed to say that the attackers act in the name of Islam, or must we continue to present all jihadist terrorists as people suffering from any affliction apart from that one?

In the middle of the week, at a memorial service in Westminster Abbey, the Very Reverend John Hall, Dean of Westminster, said that the UK was "bewildered" after the terrorist attacks of a fortnight earlier. He went on in his sermon to ask:

"What could possibly motivate a man to hire a car and take it from Birmingham to Brighton to London, and then drive it fast at people he had never met, couldn't possibly know, against whom he had no personal grudge, no reason to hate them and then run at the gates of the Palace of Westminster to cause another death? It seems likely that we shall never know."

If it is true that our societies are "bewildered", as the Dean says, might it be because we have not heard a wide-enough range of possible explanations for such outrages -- because we have deliberately cut ourselves off, by choice,- from the warnings of ex-Muslims such as Hirsi Ali? Amid the "narratives" that are acceptable and to be tolerated, perhaps we have failed to listen to the explanations that outline the sheer scale of the religious and societal problem now in front of us?

Of course, for many Muslims, such as those critics of Hirsi Ali in Australia, there is a clear reason why they want to stop her speaking. Were people to hear her, they would realise the vast enormity of the challenge ahead of us and the depth and breadth of its nature. Her audiences would discover the defensive play around the world in which many Muslim organisations are engaged -- a campaign to limit speech precisely in order to protect their own interpretation of their religion and keep out any other.

It is, however, the dissenting, silenced voices such as Hirsi Ali's that are precisely the voices the world needs to hear at present. How tragic that a week that began with a silencing, should end with yet another all-too-predictable terrorist attack -- one which Sweden will do as much to fail at comprehending as Britain did two weeks before her.

Hearing from voices such as that of Hirsi Ali could lift the fog of our "bewilderment" and explain, for instance, what does motivate some people to drive a car or truck into crowds of people going about their lives. There is a whole pile of reasons why Islamists want to stop her explanations from being aired. But why -- when the attacks keep on happening -- do our own societies collude with such sinister people to keep ourselves in the dark?