Throughout the 1960s, the tobacco industry famously spent millions promoting a small group of vociferous "sceptics" who, in the face of overwhelming evidence, continued to deny the link between smoking and cancer. The strategy paid off. Long after a clear scientific consensus had emerged, much of the public still believed that the case remained unproven.
In a sceptical age, even those disseminating wholly bogus ideas - from corporate pseudo-science to 9/11 conspiracy theories - will often seek to appropriate the language of rational inquiry. But there is a meaningful difference between being a "sceptic" and being in denial. The genuine sceptic forms his beliefs through a balanced evaluation of the evidence. The sceptic of the bogus variety cherry-picks evidence on the basis of a pre-existing belief, seizing on data, however tenuous, that supports his position, and yet declaring himself "sceptical" of any evidence, however compelling, that undermines it.
While it is easy to guess the motivations of an industry-funded scientist denying the dangers posed by his commercial sponsor, or a far-right historian expressing "scepticism" about the Holocaust, other cases are more puzzling. It is difficult to explain why, for example, a respected academic would dismiss the mountain of proof that HIV causes Aids. But several have, notably the Berkeley virologist Peter Duesberg.
HIV is a type of "retrovirus". Duesberg has argued for decades that retroviruses rarely, if ever, harm their hosts. Rather than modify this theory in the light of evidence that one such virus was killing millions, Duesberg in the late 1980s announced his "scepticism" about that evidence, and has stuck to his guns ever since.
Early on, these ideas found a receptive audience among HIV sufferers, desperate for an alternative prognosis. The cause was later taken up by conspiracy theorists convinced that Aids was a money-spinning fabrication of the global pharmaceutical industry.
In South Africa, at the beginning of this decade, Aids scepticism gained currency with a political class dismayed at the prices being charged for life-saving medicines. Under the influence of Duesberg and his fellow "dissidents", Thabo Mbeki's government chose to delay for several years public provision of anti-HIV drugs. The economist Nicoli Nattrass estimates that this decision - made amid one of the world's worst Aids epidemics - may already have cost hundreds of thousands of lives.
Bogus scepticism does not centre on an impartial search for the truth, but on a no-holds-barred defence of a preconceived ideological position. The bogus sceptic is thus, in reality, a disguised dogmatist, made all the more dangerous for his success in appropriating the mantle of the unbiased and open-minded inquirer.
Richard Wilson's "Don't Get Fooled Again" is out now, published by Icon Books (£12.99)