[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.1
If A happens, then by a gradual series of small steps through B, C,
, X, Y, eventually Z will happen, too.
Z should not happen.
Therefore, A should not happen, either.
If today you can take a thing like evolution and make it a crime to teach it in the public school, tomorrow you can make it a crime to teach it in the private schools, and the next year you can make it a crime to teach it to the hustings or in the church. At the next session you may ban books and the newspapers. Soon you may set Catholic against Protestant and Protestant against Protestant, and try to foist your own religion upon the minds of men. If you can do one you can do the other. Ignorance and fanaticism is ever busy and needs feeding. Always it is feeding and gloating for more. Today it is the public school teachers, tomorrow the private. The next day the preachers and the lectures, the magazines, the books, the newspapers. After [a]while, your honor, it is the setting of man against man and creed against creed until with flying banners and beating drums we are marching backward to the glorious ages of the sixteenth century when bigots lighted fagots to burn the men who dared to bring any intelligence and enlightenment and culture to the human mind.2
This fallacy is based upon the claim that a controversial type of action will lead inevitably to some admittedly bad type of event. It is the slide from A to Z via the intermediate steps B through Y that is the "slope", and the smallness of each step that makes it "slippery".
This type of argument is by no means invariably fallacious, but the strength of the argument is inversely proportional to the number of steps between A and Z, and directly proportional to the causal strength of the connections between adjacent steps. In other words, the longer and less slippery the slope, the weaker the argument. If there are many intervening steps, and the causal connections between them are weak, or even unknown, then the resulting argument will be very weak, if not downright fallacious.
Reader Jim Skipper writes:
I have been researching slippery slopes and wonder if you have read Eugene Volokh’s paper on the topic3. 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.
"Last month, the Princeton Regional Health Commission took a bold step to protect its citizens by enacting a ban on smoking in all public places of accommodation, including restaurants and taverns…. In doing so, Princeton has paved the way for other municipalities to institute similar bans…."―The Record (Bergen County), July 12, 2000, at L7
Not only did the PRHC slide down the slippery slope, the Record notes that it will pave the way for others to follow the same course, as we have seen throughout the United States.
Jim, I agree that Volokh's work is valuable, especially his classification of different ways in which slopes can be slippery. 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!
An eloquent example of the slippery slope fallacy. In the over ninety years since the Scopes trial, which Darrow lost, few if any of the horrors that he paraded before the jury have taken place. So, that slope apparently wasn't all that slippery.