The Prince of Wales and Al Gore may not practise as they preach, but they mean well, and that counts.

When well-known figures take a stand on global warming or the effect of fast foods on childhood obesity, they invite the press to challenge them on their personal credentials to speak out. There have been two examples of this in recent days. After winning an Oscar for his film on global warming, An Inconvenient Truth, Al Gore was met with newspaper headlines claiming that his home in Nashville, Tennessee, uses 20 times more energy than the average American household.

A C Grayling is a professor of philosophy at Birkbeck College, London