Margaret Thatcher used to ask of her colleagues, "Is he one of us?" She separated people into two categories: those with her or those against her. Over time she reshaped her cabinet, and even the civil servants working for her, to ensure that she was only surrounded by people she considered 'sound'.

Thatcher was certain of her mission and certain of her ideology. And whether you like it or not, it is undeniable that she delivered remarkable change to British society as a result.

But her certainty blinded her to criticisms, and it was her inability to compromise on the Poll Tax which caused her downfall. Certainty is the enemy of the truth. Certainty closes down debate. And there is nothing is more certain than a true believer.

True believers are necessary forces to drive change. Without the campaigning few, the rest of us usually lazily accept the status quo. The campaign to Repeal the Eighth Amendment to the Constitution is led by a campaigning few, but it has many more adherents. It's a campaign that is necessary, because the Eighth Amendment's presence has created a legal noose that prevents the Oireachtas and medical practitioners from dealing with difficult issues such as fatal foetal abnormalities, rape, and the health of the pregnant woman. And if we were waiting for politicians to drive change, we could wait decades.

But the true believing campaigners can be their own worst enemies as well.

They tend to see things as Thatcher did, ideologically. Ideology pushes people to think in binary terms - things are right or wrong. On one side we hear that abortion is murder, on the other we hear nothing should interfere with a woman's right to choose.

But most people hold a position in between these. If you were to dig deeper you'd find almost no one agrees to the absolute right to procure an abortion. Few would permit an abortion of a viable foetus after 26 weeks. Equally, a vanishingly small proportion of the population oppose abortion in all circumstances. So people want something in between.

The same is true of immigration. Most people in Ireland are welcoming of immigration. Data from the Irish National Election Study show 70pc of people disagree with the statement that "Immigrants damage Ireland's culture." And 70pc agree that immigrants are good for the Irish economy. But having no limits or rules on immigration would be unviable. Almost 80pc of people also want minorities to adapt to Ireland's way of life. And 55pc of Irish people agree that there should be "very strict limits on the number of non-EU migrants coming to Ireland". Fewer than 8pc strongly disagree with that statement, yet we never hear any debate on what reasonable limits might be.

In countries where debates on the issue of immigration have started, they have been started by the extremes. The problem with campaigns is that they push you into one of the two tribes. Many people feel uncomfortable with either label. If they try to question assumptions, either tribe makes them appear to be declaring: "I'm not in your gang."

The true believer takes any questioning or criticism personally. You are denying the experience of the woman who has had an abortion or the religious beliefs of the opponent of abortion. You are part of the liberal metropolitan elite or you don't appreciate the hardship suffered by immigrants.

Most people don't want to appear rude, so rather than engage they withdraw from the debate. A few months ago, in reaction to the demand for 'free, safe and legal' abortions, I ventured the opinion on Twitter that abortion should be safe and legal, but asked why it should be free. It seems to me that in cases where pregnancy is not a threat to the health of a woman, there is no reason that taxpayers should fund abortions. I was roundly condemned for my 'neoliberal' views. I withdrew.

If campaigns end up alienating people on their own side - as I think I am - imagine what it does to those who are uncertain. It becomes much more difficult to reach them. Even opinion polls on the topic might be less accurate if people are afraid to venture opinions.

What happens then is that only the true believers get to talk to each other. The true believers rarely display any understanding or respect for their opponents' position. Neither side is morally dangerous or evil, but both sides see the other in these terms.

The social psychologist Jonathan Haidt says this self-righteousness comes from a failure to appreciate the different moral foundations for the other side's beliefs.

When these 'righteous minds' debate, we escalate the rhetoric. It happens easily. You start off with the best of intentions, but when one side accuses you of something, you hit back. I experienced it when I campaigned for the abolition of the Seanad. In response to what I thought were fanciful suggestions that abolition posed some sort of threat to our democracy, I suggested the Seanad was some sort of threat. It was nonsense; the Seanad will never harm or help anyone.

For Una Mullally, the abortion debate can be reduced to "Catholicism versus women's rights, religious ideology versus fact". The vandalism of Mick Barry's constituency office with the slogan "Baby Killer" is another example of such binary thinking.

These attacks make the debate fun to watch, but unpleasant to be in. But they also give the impression that the country has polarised when in fact it hasn't. On most issues, people - whether in Ireland, the UK or US - sit in the middle.

The escalation of the debate and the withdrawal of the centre means we don't have the debate. And because centrist politicians are afraid to step into the debate on abortion or immigration, we see others step in to fill that void. It is the silence of centrists that leads to Le Pen, Trump and their ilk convincing people that their problems are caused by immigration, when they might be caused by education, housing or healthcare.

Dr Eoin O'Malley is director of the MSc in Public Policy at Dublin City University

Sunday Independent