It is the burden, if not the tragedy, of Bradley Wiggins and all those against whom he competes in the Tour de France that he knows deep down there is one race he can never win. It is the one against doubt, the opponent whose presence is always most tangible in the wake of the most brilliant deeds and conspicuous courage.
If you are clean you rage against the suspicion that your greatest performances inevitably come with chemical assistance, as Wiggins did in a volley of expletives the other day. The trouble is there is simply not enough of the stuff to go around – rage, that is. This is because it is employed as fiercely by the guilty as the innocent.
It means that the 32-year-old Wiggins, and all who share his brutally demanding trade, have an obligation that is more easily discussed than performed when you are awash with adrenalin, when you are pushing yourself into an effort that in his case might well reward him with the first British triumph in the greatest, most demanding race that has ever been devised.
However sad and dispiriting it may be, the requirement is to understand that a presumption of innocence can never again be carte blanche.
There has been too much systematic corruption for that, too much cheating on an industrial scale, and if most of us can recall the time when our own doubts became part of our reactions to the highest achievement in certain sports – maybe it was Ben Johnson at Seoul or Marion Jones in Sydney – in cycling there is one particularly vivid memory flash.
It is of the Belgian Michel Pollentier at the peak of Alpe d'Huez in 1978, uncovered after riding the shift of his life, a magnificent ascent that filled the mountainside with cheers as he shattered the field and claimed the yellow jersey. It was entirely by chance that he was detected.
Another rider at drug-testing aroused the suspicion of the presiding doctor as Pollentier wrestled with tubing designed to deliver uncontaminated urine. The Belgian, who the previous year had won the Giro d'Italia, was asked to remove his jersey. He had a condom filled with clean urine attached to a tube running down from his armpit.
The long-running charges against Lance Armstrong and so many of his successors concern far more sophisticated methods, of course, but they are founded in the same suspicion that when the challenge is so hard, and the awards are so great, someone will always be prepared to gamble on the possibility of life-changing success.
For Wiggins the implications may be unpleasantly insulting but, unfortunately, they are part of the terrain he so superbly conquered this week in the 41.5km (26-mile) time trial which may just have given him permanent possession of the yellow jersey.
Like Usain Bolt in Beijing four years ago, Wiggins can only bask in the acclaim of all those who are still prepared to believe that the purest achievement can still come, well, purely.
When Bolt came home in Beijing in his astonishing time, in his astonishing gait, and the eyes of the world flashed from the track to the electronic board that announced a new record time, for a moment it was as though you were back in Seoul in 1988, when Johnson made his own impact on the art of the possible. There was the same swivelling of heads, the same need to believe.
If it should happen that Wiggins, already the owner of three gold medals, arrives at the start of the Olympic road race in three weeks as the winner of the Tour de France – we can only believe that both belief and national pride will be running on an extremely high tide indeed.
This is a potentially superb, unprecedented achievement and who would not want to embrace the sentiment of the hero's brilliant ally – at least until they reach the most draining cols – Chris Froome, when he declares: "Cycling has evolved. Dedication and sacrifices = results. End of story"?
Who would not say amen to that? Who would not celebrate the end of all doubt and that the bravest of athletes receive the most uncomplicated acclaim when they make the peak and go rushing, whooping and whistling down towards a glorious chase for the finish?
It has long been a contention here that nothing is more moving in all of sport than such a sight and it is one enhanced if you ever happen to be in the bare hotel room of one of the riders near the end of their ordeal, when their bodies are breaking down and erupting with boils and when a team masseur works to take away some of the bone-deep ache.
Mark Cavendish described the interior of such a room, where he spent some of yesterday's rest. It had no more luxury than an electrical outlet and a plastic chair.
They are the toughest of men who spend their lives between the peak and the abyss and it is perhaps appropriate that the time trial discipline in which Bradley Wiggins may have sealed his greatest prize has always been known as the race of truth.
The winning of such a race, in every respect, is not only a superb ambition but the answer to a sometimes despairing prayer.
The implications may be unpleasantly insulting but, unfortunately, they are now part of the terrain