Robert Tyre Jones Jr was the greatest amateur golfer of any era. He entered only 21 tournaments and won 13 of them. But it is for his sportsmanship that Bobby Jones is most remembered. In a play-off at the 1925 US Open, he nudged his ball as he prepared to hit it. Jones called over the marshals and announced that he had hit the ball twice. No one had seen anything amiss, but Jones was insistent that he must be given a one-shot penalty.

He subsequently lost the tournament by one stroke. When praised for his sportsmanship, Jones replied: “You may as well praise a man for not robbing a bank.”

Jones was a moral man from a different era playing a high-minded game. But not all sports have had a similar attitude to cheating. Ty Cobb, a contemporary of Jones who was about as good at baseball as Jones was at golf, sharpened his spikes on the ground before going into bat, shouting his intention to gouge the opposition infielders if he got the chance.

It is often argued that cheating is getting worse and sportsmanship is declining. But one fact often ignored is not only that rules change, but also that conventions evolve. In cricket, not so long ago, most batsmen (in theory anyway) claimed to “walk” - in other words, if they knew they had nicked a catch to the wicketkeeper, they did not wait for the umpire's decision. Only recently has “standing”, when you know you have edged the ball, become typical behaviour in the first-class game.

And yet in rugby, until very recently, there was only one referee - the touch judges did not help with disciplinary issues, only line calls - and the odd flurry of punches among the forwards was considered all part of the sport's rough and tumble. In the mêlée, who could be sure who did what? Best to “let the game flow” because it was “all part of the game”. But in rugby today fewer punches go unpunished.

Rugby, though no longer amateur, is a cleaner sport. The boundaries of fair play, in other words, are constantly moving, and not always in the “wrong” direction. Above all, sport is riddled with inconsistencies. You can break some rules with impunity, but if you break others you are a barefaced cheat. Unpick those riddles and you get into interesting territory.

“Walking” in cricket is a case in point. My father's generation of Englishmen was brought up to walk. I was not. But in one of my first championship matches, against Somerset, I nicked a bat-pad catch and “walked” - more out of instinct than morality. It felt out and I just walked off, almost without thinking. David Shepherd, the umpire, called me over after the game and said: “Well done, lad.” Some in the Kent dressing-room took the opposite view.

Most modern professionals (and amateurs, too) think that the convention is not to walk but to accept the umpire's decision. The theory is that decisions “even out” over the course of a game and the authority of the umpires is upheld. But, under the same set of conventions, it is not considered acceptable to scoop up a half-volley at slip and claim it as a catch. So it is not cheating if you are 100 per cent sure you edged the ball but do not walk, but it is cheating if you are 100 per cent sure the ball bounced but you claim the catch.

Attitudes to cheating change imperceptibly. The professional foul is another example. It has always been a feature of sports such as basketball, where each player has a quota of five fouls he is allowed to commit before being fouled out. It is considered acceptable, and within your rights, to grab hold of an opponent who is about to score.

Such a view may seem anathema in rugby union. A good haymaker here and there is one thing, but premeditated spoiling of the game is surely another. Not really. How many of the great flankers, in searching for the one or two-yard headstart, spent most of their careers offside? Nothing wrong with that, most former players would say, you play to the referee. But how about blocking someone's run in the backs? The only difference is that the latter is more blatant. Is obviousness how we determine our code of ethics?

Arguments about cheating in sport revolve around conventions more than laws. It is not only a question of “was he breaking the rules?” but also “is that rule sacrosanct?” It is the unwritten constitution that exerts the stronger grip. A case in point was the controversy that surrounded the Test match between Pakistan and England at the Oval in August 2006. Darrell Hair, the umpire, penalised Pakistan five runs for tampering with the ball. Pakistan were so offended that they refused to take the field after the tea break - at which point Hair removed the bails and declared that Pakistan had, by their actions, forfeited the match.

And so chaos turned into crisis. The match was abandoned, but would Pakistan also abandon the tour? Had their honour as a team, perhaps even a nation, been grievously offended?