Why we should avoid the ‘slippery slope’

The phrase ‘slippery slope’ is used all the time in public debate, but the argument behind it is a fallacy

slippery slope
 The slippery slope was originally more or less the metaphorical equivalent of what we now call a greasy pole. 

Public debate is a risky business: everyone is in danger of plummeting cartoonishly down icy inclines. That, at least, is the picture evoked by the constant invocation of a “slippery slope”. Commentators recently said that Clean Reader, the app that bowdlerises ebooks, puts us on a slippery slope towards total literary censorship. During the debates about mitochondrial transfer, we heard that it was on a slippery slope to “designer babies”, or grafting the heads of lizards on to defenceless infants. And every time voluntary euthanasia comes up, people denounce it as a slippery slope to widespread murder by doctors of old people they find annoying. Legalising cannabis is a slippery slope to the entire population spending all day off their faces on acid. And so on.

In the field of informal logic, the slippery-slope argument is a fallacy when the endpoint does not follow necessarily from the initial step, which is especially obvious if there are many (unidentified) intervening stages involved. In debates over legislation, it is even more clearly fallacious, because laws are instruments that explicitly allow some things but not others. If we pass a law allowing mitochondrial transfer, that does not permit us to enact whatever science-fictional baby-customisation can possibly be envisaged.

In point of social and political fact, there is enough friction on the slope to enable us to stop wherever we choose. The invocation of a slippery slope is therefore almost always anti-rational fearmongering. The phrase acts as a little weapon of ready-made false reasoning that the wielder supposes to function as a knockout argument. We could call this kind of thing “Unthink”.

The most sensible thing that can be meant by an appeal to slippery slopes is not a truth about the dynamics of public policy, but rather a worry about our moral psychology. The idea must be that, if we all get used to this one new thing being allowed, we will be less able to resist a slightly more alarming thing in the future. And so, through repeated exploitation of our piecemeal moral relaxation, we will all end up living in an apocalyptically permissive Gomorrah. If this is true, however, I for one don’t understand why there are still things I’m not allowed to do. After all, slippery slopes have already been invoked for more than a century of political change.

The Oxford English Dictionary dates the metaphorical slippery slope back only to 1951, which is surprisingly recent. But an antedating is possible: several earlier sources mention that it was a favourite image of the 19th-century archbishop of Canterbury Frederick Temple, who used it in writing in 1895. It’s probably impossible to know whether Temple was using the phrase in conversation before it appeared in the 1887 novel Wee Wifie by Rosa Nouchette Carey, where it is lamented: “Alas, Hugh Redmond was not the only man who, stung by passion, jealousy, or revenge, has taken the first downward step on the green slippery slope that leads to Avernus.” Avernus, literally the place with no birds, was a stinking Italian lake considered in classical times to be an entrance to the underworld. So this early use of the image hovers between metaphor and literalism: sliding down grass to a sulphurous doom.

It seems, though, that the slippery slope as one on which we slide down to moral depravity came about via a simple reversal of direction. An earlier example suggests it is a slope one desperately scrambles up. In George Croly’s Marston: Or, the Memoirs of a Statesman, serialised in the May 1844 issue of Blackwood’s Magazine, we find it written: the “doctor still wrote verses, and still had a lurking propensity for climbing the slippery slope of poetic renown”.

Slippery slope was originally more or less the metaphorical equivalent of what we now call a greasy pole: something worth trying to ascend, even if one often slides back. Perhaps next time we hear some bloviating windbag complaining that a proposed policy is on a “slippery slope”, we can respond: “Yes, but I think we can get there if we really try!”



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