I DON'T believe in astrology but many people do. About half the women I meet ask me my star sign. I used to try to explain why they shouldn't believe in it but I have given up. They can never answer my objections, but nor can I change their minds. They usually just get angry with me. Many even suggest that I am attempting to violate one of their rights: namely, their right to their own opinion.

It isn't only astrology enthusiasts who insist upon their right to believe whatever they like. Type “I am entitled to my opinion” into a Google search and you will see that it is a standard riposte of the frustrated debater, on topics as diverse as politics, religion, music and football.

The idea that everyone is entitled to his opinion is one of those truisms so often repeated that it now goes without saying. Like many truisms, however, it is false.

It is also usually irrelevant. Let us suppose that Jill disputes Jack’s opinion that free trade causes poverty in the Third World. Jack may defend his opinion by producing evidence connecting trade and poverty but he cannot help his case by insisting that he is entitled to his opinion. How could that show that free trade causes poverty in the Third World?

The entitlement would be relevant only if it guaranteed the truth of your opinions. But it can’t do that, because it is an entitlement supposedly enjoyed by everybody. And people disagree. Jack and Jill are both entitled to their contradictory opinions about trade and poverty, but they can’t both be right. So insisting that you are entitled to your opinion cannot possibly give you any proper advantage in a debate.

Especially since there is no such entitlement in the first place. We do not have a right to our own opinions.

To see this, we need only understand one basic point about rights: namely, that rights entail duties. I don’t mean to endorse the new Labour slogan “No rights without responsibilities”, which is supposed to justify policies whereby the Government imposes good-behaviour conditions on the receipt of social welfare. I mean something much more fundamental about rights: they are defined by the duties to which they give rise.

The law gives us all a right to life. Your right to life means that everyone else has a duty not to kill you. This is not something that the Government may or may not decide to associate with your right to life; it is that right. A law that did not impose on others a duty not to kill you would thereby fail to establish your right to life.

Does your right to life mean that others have a duty to feed you, to house you, to provide you with medical care? These are hotly debated questions, but no one doubts that the answers to these questions about others’ duties are what define and delimit the right to life.

So when anyone claims a right, the first question to ask is what duties this right is supposed to impose on others; that will tell you what the right is supposed to be. It also provides a good test for whether there is, or should be, any such right. For it will often be clear that no one really has the implied duties, or that it would be preposterous to claim they should.

I once heard an Australian government minister claim that every child has a right to be loved. But who could possibly have a duty to love every child? Or even a duty to love a single child? Of course, it would be nice if every child were loved. But that is irrelevant. That something would be nice to have — such as long eye-lashes or £10 million — does not mean that anyone has a duty to provide you with it. Nor, therefore, that you have a right to it.

What, then, are the duties that the right to your opinions might entail? What am I obliged to do to respect this right? Let’s start from the boldest possible demands and work down to the more humble.

Does your right to your opinion oblige me to agree with you? No, that would make the duty impossible to perform. For I too have a right to my opinion, which you must respect. If we disagree, I must change my opinion to yours, and you must change yours to mine. But then we disagree again, and must change our opinions again. And so on forever, never managing to do our impossible duty.

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