What for one person may be an inconvenient truth may for another be very convenient indeed: bad news about your plumbing is good news for the plumber. Yet when it comes to our values, we're much better at pointing out truths that are bothersome for others than we are spotting those that make life awkward for ourselves. Indeed, we go further, and deny inconveniences that those with clearer intellectual vision would spot a mile off.

Last night, for example, supporters of Compassion in World Farming (CIWF) were no doubt pleased to hear Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change (IPCC), tell them that reducing meat consumption was essential if we are to arrest global warming. Meat production is responsible for 18% of the world's greenhouse gases, as compared to just 13% for transport.

It sounds like a wonderful confluence of green and animal welfare concerns. However, look more carefully and it turns out that the fit is not so neat after all. Some of the best ways of reducing methane emissions for cows, for example, involve stopping them freely grazing on grass and raising them in sheds, on feeds that produce less gas. Other options include genetically engineering breeds of cattle that are less flatulent. The inconvenient truth here is that, although many compassionate and green farming practices are also better for the environment, not all are. I find it incredible how far people will go to deny such truths. People seriously argue that, for example, organic farming is always better for the environment than conventional farming, in every way. Facts that contradict this will be dismissed and those that seem to support it paraded, when what should happen is that all the relevant facts be examined dispassionately.

To think that one's goals and values never conflict with one another is not just irrational, it's immature.And it's important that we get used to this uncomfortable reality, because if we don't, our fixed preconceptions can stand in the way of achieving the goals we value the most. For example, a cross-party MPs' report, published yesterday, argues that we need zero net immigration. For someone like myself, who likes to think the (reasonably) free movement of people makes for a better world, this is very challenging. Worse still, I care very much about the lamentable way we often treat asylum seekers, and I know that tough messages on migration tend to go hand in hand with tough lines on asylum.

My knee-jerk reaction may yet prove to be right. At first glance, the report, Balanced Migration, has some questionable aspects, not least the involvement of its publisher, Migration Watch, which hardly comes to the issue with an open mind. Nevertheless, we need to take seriously the possibility that something like the proposal may indeed be necessary.

If we don't, and the MPs are right, then ignoring their advice will only exacerbate the frictions that make people hostile to asylum seekers. In refusing to accept truths discordant with our current beliefs, supporters of asylum seekers could end up doing the cause we most believe in harm, not good.

But if the case is sound, accepting it might actually help improve the lot of asylum seekers. If net immigration is zero, then the issues of asylum and immigration can more easily be kept separate, as they should be. People would be much more willing to defend the right to asylum if they knew that it would not lead to "Britain being flooded".

In his film, Al Gore quoted Upton Sinclair: "You can't make somebody understand something if their salary depends upon them not understanding it." That's true, but we need to remember people who work for lobby groups and charities have salaries, too, and that wages are not the only things we value. A deeper, broader version of Sinclair's maxim would be, "You can't make somebody understand something if their existing world view depends upon them not understanding it."

We all have ways of seeing the world which might cause us to irrationally resist truths discordant with them. That's why we all need to recognise what it is convenient for us to believe, and make sure that convenience is not the reason why we believe it. Julian Baggini's latest book, The Duck That Won The Lottery and 99 Other Bad Arguments, is published by Granta.