[I]f once a man indulges himself in murder, very soon he comes to think little of robbing; and from robbing he comes next to drinking and Sabbath-breaking, and from that to incivility and procrastination. Once begin upon this downward path, you never know where you are to stop. Many a man has dated his ruin from some murder or other that perhaps he thought little of at the time.
Source: Thomas De Quincey, "Second Paper on Murder"
There are two types of fallacy referred to as "slippery slopes":
Though these two fallacies are distinct, and the fact that they share a name is unfortunate, they often have a relationship which may justify treating them together: semantic slippery slopes often form a basis for causal slippery slopes. In other words, people often think that a causal slide from A to Z is unavoidable because there is no precise, non-arbitrary dividing line between the two concepts. For instance, opponents of abortion often believe that the legality of abortion will lead causally to the legality of infanticide, and one reason for this belief is that the only precise dividing line between an embryo and a newborn baby is the morally arbitrary one of birth. For this reason, causal slippery slopes are often the result of semantic ones.
A great deal of ink has been spilled in fruitless philosophical debates over exactly where to draw the line between concepts that lie on continua. This might be called the "legalistic" side of philosophy, for it is primarily in the law that we are forced to decide hard cases that lie in gray areas. For instance, if the legislature were to decide that baldness is a disability deserving of certain benefits, then the courts might be forced to decide the issue of whether a certain person is bald. In everyday life, we are seldom faced with decisions of this kind, and we continue to use the concept of baldness without worrying about borderline cases. When someone falls into the fuzzy area between bald and hairy, we just say that he is "balding", thus avoiding the issue of whether he is now bald.
One reason that so many philosophical debates are seemingly endless and undecidable is because they involve a search for a mythical entity, namely, a non-arbitrary distinction between concepts which lie upon continua in conceptual space. The logical attitude towards such problems is to avoid them if at all possible; but if a decision cannot be avoided, then draw an arbitrary line in the gray zone and stick with it. Don't be drawn into defending the decision against the charge that it is arbitrary; of course it's arbitary, for any such decision will be arbitrary. For this reason, it is not a criticism of such decisions to point out their arbitrariness. Philosophers, naturally, are uneasy about arbitrariness, but when we are dealing with conceptual continua, it is an unavoidable fact of life. Where there is only gray, there are no black-and-white distinctions to be made.
Reader Jim Skipper writes:
I have been researching slippery slopes and wonder if you have read Eugene Volokh’s paper on the topic. Mr. Volokh demonstrates mechanisms by which slippery slope progressions take place in real world situations. While, granted, slippery slope arguments are sometimes fallacious, when dealing with people and group behavior, logic and reason seldom apply. I point this out because Mr. Volokh’s paper is very impressive and he gives a beautiful, real-world example of a slippery slope in action, in which someone assures the public that a slippery slope will not happen and then it does:[Regarding legislation to outlaw cigarette machines.] "Sandra Starr, vice chairwoman of the Princeton Regional Health Commission…, said there is no 'slippery slope' toward a total ban on smoking in public places. 'The commission’s overriding concern,' she said, 'is access to the machines by minors.'"―New York Times, Sept. 5, 1993, § 1, at 52.
Jim, I agree that Volokh's work is valuable, especially his classification of different ways in which slopes can be slippery. You'll notice that I had already included a link to his short paper in the Resources listed above, and I've added a link to the long version below.
The fact that I list the causal version of the slippery slope as a fallacy does not mean that every argument with the form of a slippery slope is fallacious; rather, it means that sufficiently many are fallacious to make it worth including as a type of common logical error―that is, a fallacy.
However, I suspect that I'm more skeptical of the slipperiness of slopes than Volokh. While we agree that some slippery slope arguments are cogent and others are fallacious, Volokh seems to think that more are cogent than I do. One reason why I am skeptical has to do with the difficulty of the causal reasoning needed to establish that a slope really is slippery; most slippery slope arguments make little or no attempt to do this hard work. Moreover, it is difficult even in retrospect to tell whether a slippery slope mechanism has actually been at work. This brings me to the example that you cite.
In order for the cigarette machine example to show a slippery slope effect, the law banning cigarette machines must have contributed causally to the subsequent ban on smoking in public places. However, from the two newspaper quotes given, all that we know for sure is that the machine ban preceded the smoking ban. To reason that when one event precedes another the former causes the latter is to commit the post hoc fallacy.
Volokh is in fact aware of this problem, as he writes:
I would have liked to illustrate the discussion with case studies of how the legal system has slipped down various slippery slopes, but unfortunately it’s generally very hard to tell whether legal change A in fact caused legal change B (even if it’s plausible that it did),…such case studies might therefore have become more controversial than persuasive. (p. 1039, footnote 39)
That's another thing that Volokh and I agree on!
Source: Eugene Volokh, "The Mechanisms of the Slippery Slope", Harvard Law Review 116 (2003), pp. 1026-1134.
An eloquent example of the causal slippery slope fallacy. In over seventy-five years since the Scopes trial, which Darrow lost, few if any of the horrors that he paraded before the jury have taken place.
Thanks to Kevin Donaldson for a criticism which led to some revisions to the section on the Semantic Version of the fallacy.