After the anti-immigration fanatic Anders Behring Breivik killed 77 people in Norway, Daniel Cohn-Bendit, co-president of the Green bloc in the European Parliament, asserted confidently, "A lot of arguments about immigrants and Islamic fundamentalism will now be much easier to question and to push back." There seems to be a consensus that this grotesque mass murder in such a peaceful, low-crime country has seriously damaged the case for stricter border control and reduced immigration in Europe.

Ad hominem attack is often carelessly conflated with the use of insulting or discourteous language. Rather, the fallacy entails discrediting an argument by discrediting its advocate instead.

Left-wing reaction to Norway's misfortune has engendered a subtle variation on ad hominem tactics. In committing an act universally regarded as horrifying, irrational, and repugnant, Breivik effectively discredited himself. Even right-wing European parties have bent over backwards to disassociate themselves from this killer. The question is, then: did Breivik therefore discredit his political position as well?  To claim that the Norwegian massacre proves anything whatsoever about the argument against mass immigration is ad hominem reasoning.

While we're throwing Latin around: in attempting to make Norway's massacre a political turning point, European leaders have flirted with another logical fallacy, post hoc ergo propter hoc. Sigmar Gabriel, the head of Germany's Social Democratic Party, asserted in July that with the proliferation of anti-immigration dogma, "naturally on the margins of society there will be crazy people who feel legitimised in taking harder measures." Observe the fallacious causal leap: Breivik held opinions, in the pursuit of which he was violent; therefore these opinions lead to violence. By blaming the ideology of rising anti-immigration parties for Europe's worst single-gunman atrocity in modern times, continental leaders have employed the same opportunism as some Labour politicians, who blamed August's free-iPad riots on Tory cuts.

It's foolhardy to allow whole arenas of debate and lines of thought to be colonised by the loopy fringe. For decades, immigration was off-limits for decent, democratically-minded people, since even gently to question the number of migrants flooding into the West was necessarily racist and xenophobic. We've made some progress in carving out the right to consider the social and economic consequences of high rates of immigration without tarring every sceptic of the-more-the-merrier multiculturalism with a bigoted brush. Norway threatens to shove the issue back into the closet.