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.

Likewise Prince Charles's critical reference to McDonald's, while visiting a health centre in the Arabian Gulf, prompted newspapers to reveal that his own Duchy Originals Cornish pasties contain more fat, salt and calories than a Big Mac. The implication is that because Gore and the prince do not do as they tell everyone else to do, at best they undermine their own message and at worst deserve the name of hypocrite.

Neither the prince's pasty nor a Big Mac is especially bad on a one-off basis. A

nd Gore gets his energy from renewable sources, so even though his life of public commitment doubtless leads to him burn far more energy in the use of communications and computing equipment than does the average household, he can legitimately claim to be green nevertheless.

Both men are obviously well meaning, and their campaigns for environmentally sensitive alternatives to consumerist depredations on the planet are important and timely. If they fail to be greener than thou, does that impugn their message? To think so is to commit the ad hominem fallacy, in which the truth of what someone says is taken to be undermined by the fact that he does not act accordingly. But clearly, although saying one thing and doing another makes a person inconsistent or, when the aim is to cheat in some respect, a hypocrite, it does not automatically falsify what he says.

There is such a thing as "doing one's moral best", which may, and usually does, fall short of perfection but nevertheless constitutes a serious gesture in the right direction. Take the person who is vegetarian on the grounds that he thinks it morally undesirable to kill and eat other sentient creatures, but who owns leather shoes and belts. This latter is certainly inconsistent with vegetarianism, but the person might reason that being a habitual meat eater makes a vastly greater "slaughter footprint" than buying a pair of leather shoes, and, moreover, to be completely strict in his practice demands a degree of time and effort - given that he has other calls on both - not justified by the results. So he takes a stand short of the logical limit of his commitment, and trusts that it will make some degree of difference towards the good nevertheless.

Doing one's moral best, where this is understood to be less than the true best, is open to abuse and serves as a cover for hypocrisy. It may even do so more often than it sincerely balances practicalities with a desire to go in the right ethical direction. Yet this kind of sincerity really is possible, and is at work in the thought that if everyone did at least something - about recycling, say, or in offsetting carbon emissions caused by their air travel - the cumulative effect would be great. It is better that people do something towards the good rather than nothing.

That "something" is sure to be far less than individuals could do - for example, in making their homes and lifestyles green. There would remain room for improvement. But that is acceptable, because for most people the all-out effort is unsustainable, so the pragmatic choice is for them to seek a reasonable balance between this concern and their other legitimate and doubtless personally significant commitments and avocations.

The alternative is to say that unless a person achieves the utmost, let him do nothing; which is the same as letting him be careless and indifferent. Obviously we would rather he made some degree of effort, as much as he could without turning what he does into a penance. We are admitting that doing one's moral best is a legitimate and worthy compromise, and we thereby admit that Al Gore and Prince Charles can be applauded for standing up for the right causes even if we find they are not much different from the rest of us in the progress they have made in their own lives. That does not mean we cannot expect them thereafter to smarten up their act, but if we are going to point the finger at them for not doing what they say we should, does that not imply that we accept they are right? The editors of the first newspapers to publish facts about Al Gore's energy consumption and Prince Charles's pasties should themselves be audited on their energy footprints and their dietary habits, so that we can measure the height of the moral ground on which they stand.

Doing one's moral best is close to Aristotle's idea that, in effect, one lives an ethically good life by trying to do so. To try is to succeed, otherwise the only good people would be perfect people, and all those striving to do their moral best would not be good people. The notion that it is the trying which is the succeeding is not paradoxical, but realistic - realism and truth to life are two of the great virtues of Aristotle's ethics. He talks of forming "habits" of virtue, where "virtues" are character traits that achieve a middle course between opposing vices; thus courage is the middle way between cowardice and rashness, and generosity is the middle way between meanness and profligacy. By always trying to recognise the middle course between opposite failings, said Aristotle, using their practical intelligence and experience in the process, people form the desired moral habits. It is these that, together, make a person, and his life, good.

The Greeks had an interestingly different view of moral failure from the later and more widespread Christian view. According to the latter, to do wrong is to sin, and a sin is a stain on one's immortal soul, requiring redemption, cleansing, expiation, forgiveness. "Sin" means, literally, disobedience to divine command. The Greeks, by contrast, gave to moral failing a name which literally means "a bad shot," as when one fires at a target and misses. The remedy is to try again, and do better next time. This robust and healthy view is forward-looking, constructive and positive.

Much of the traditional idea of sin persists in our contemporary attitudes to moral failure. We somehow export the idea of a stain, an enduring flaw of character, to the case of people who do not live up to ideals, especially those they themselves proclaim. This is where a hostile media can inflict a body blow not only on the likes of Al Gore and Prince Charles, but on what they are trying to do, because in a sin culture even the suspicion of hypocrisy in the messenger is enough to harm the message: if the source of the claim is polluted, the claim itself must be questionable. In the Greek view, the value of what is said and the character and actions of the person who says it are separable, and can be independently evaluated on their merits. None of this is intended for a moment to excuse or minimise hypocrisy or deliberate inconsistency; people can legitimately be caught out. The important contrast is with people trying to do as much of the right thing as is feasible for them without claiming, or even aspiring, to be paragons.

Throughout history earnest moralisers have stood in the way of the good by accepting nothing less than the utmost. Human beings are a mixed alloy: the same person is capable of being good and terribly bad at different times or in different respects. That fact unites the greatest moral philosophers - these being novelists and dramatists - in insisting that we should resist thinking that anyone is wholly one or the other, even at their best or worst moments respectively.

I would rather have an energy-wasting Al Gore fighting to save the planet than an energy-wasting Al Gore not caring about the planet. People such as Gore and Prince Charles have a platform, and the worst thing they could do is fail to use the platform in support of worthwhile causes, whether or not they are personally no better than the rest of us at doing their individual bit.

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