I wonder if I was the only one who noticed the interesting juxtaposition of stories on Saturday. First, an Australian Holocaust-denial group losing a lawsuit in the federal court and being ordered to remove material from their website. Second, a CSIRO scientist saying, in the wake of the Intergovernmental Panel on Climate Change report, that “the global warming debate is over.”

Now let’s agree straight away that Greenhouse denial is not in the same league as Holocaust denial. There are still decent, honest and intelligent people – albeit a small and diminishing number – who are sceptical about the Greenhouse effect. There are none fitting that description among the Holocaust revisionists.

Nonetheless, the difference between the two is fundamentally a matter of degree. Both raise questions about the limits of free speech, and indeed about just what free speech involves.

Greenhouse-deniers allege that their complaints are ignored or censored by the scientific establishment and the mainstream media. But of course anyone can make that claim. It just isn’t possible for us to take everyone’s beliefs with equal seriousness, no matter how off-beat, discredited or downright loopy they might be. If we try, we will end up having to admit Holocaust-deniers on an equal footing with genuine historians.

All that can be asked in the name of free speech is that the views of cranks should not be forcibly suppressed. On that score, the Greenhouse deniers have no cause for complaint.

But the Holocaust-deniers have. The attempt to criminalise their opinions is not only wrong in principle, but counter-productive. Few would ever have heard of the “Bible Believers” if the Executive Council of Australian Jewry had not chosen to take them on in court; now their hate-filled message will gain more attention, and some will suspect that their opponents must have something to hide.

Similarly, if in ten years’ time Greenhouse denial has the same sort of intellectual standing that Holocaust denial has now, that will be no excuse for threatening to send its proponents to jail. As Thomas Jefferson said in a similar context, “let them stand undisturbed as monuments of the safety with which error of opinion may be tolerated where reason is left free to combat it.”

For a third example, somewhere between the two, consider HIV denial, currently at issue in a court case in South Australia. Free speech demands that HIV deniers be free to express their views.

But the rest of us must also be free to decide that those views are beyond the range of respectable opinion. They can talk, but we don’t have to listen.