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The slippery slope is a common logical fallacy (and a variant on the argument from adverse consequences) that asks for a prohibition or curtailment on something based on a cascading series of undesired results. While the slippery slope may in fact exist, usually the argument is put forth while ignoring possible mitigating factors, thus making it generally a somewhat-too-serious form of reductio ad absurdum.
The fallacy is a conditional fallacy.
 Alternate names
- domino effect
- argument of the beard
- fallacious reductio ad absurdum
P1: If A, then sometimes B.
P2: If B, then sometimes C.
P25: If Y, then sometimes Z.
C1: If A, then Z.
P1: X happens.
P2: Y happens to some people that do X.
C1: Therefore, Z will happen to all people that do X.
The two primary forms of the slippery slope argument are the logical form, (in which acceptance of A must logically lead to acceptance of the undesirable B), and the psychological form, in which it is argued that the acceptance of A will, over time, lead people to be more willing to accept B. The logical form is further broken into two basic models. In the first, acceptance of B is a logical extension of A. The second, on the other hand, is known as the "arbitrary line" approach, in which it is argued that the acceptance of A will lead to the acceptance of A1, because A1 is not significantly different from A. This, in turn, will lead to A2, A3, and, eventually, to B. The first version can be shown to be logically valid if it can be shown that A logically entails B. The second version is only valid if it can be demonstrated that there is no clear point of demarcation between any two of the steps.
 Parade of the Imaginary Horribles
The parade of the imaginary horribles is the list of off-the-wall worst-case scenarios that opponents claim will occur after the legalization or acceptance of a prohibited activity (e.g. Santorum's "man-on-dog sex" statement listed below). These scenarios say more about the psyches of the people who devise them than about the proponents for change.
This term is attributed to former U.S. Supreme Court Justice William O. Douglas.
The slippery slope argument may have some validity when the matter under discussion is a proposed political compromise with an opponent who has an agenda with many more items on it besides the one on the table. This is usually called appeasement; the underlying argument is that agreeing to the proposed "compromise" will not buy you peace, but only leaves you in a weaker position when confronted with the next demand.
Former Senator Rick Santorum's belief that the US Supreme Court's decision on Lawrence v. Texas would eventually lead to the legalization of "man-on-dog" sex, among other forms of sexual activity generally considered repulsive or obscene.
It is also a common argument amongst opponents of drug or alcohol use, based on the idea that simply getting drunk or high once (or even just experiencing a little of the substance in question) inevitably leads to a raging addiction and a failed life.
Logical fallacies are a dangerous thing.
Avoid logical fallacies at all costs; there's no telling where they'll lead!
This is an example of the casual form:
- 'If I make an exception for you, I'd have to make an exception for the next person. Pretty soon, the rules would be meaningless.'
While this may not always be fallacious, sometimes A and B (i.e. making one exception and making many) are distinct enough that the argument becomes very weak. In this example, it is in fact possible to make an exception in one case only, as in an emergency, but not in usual cases.
The semantic form relies on the second of the two logical formulations of the slippery slope, as opposed to the psychological version. As an example of the semantic form:
- 'Remove one grain from a heap of sand, and you still have a heap. Remove another - still a heap. No matter how many you remove, you still have a heap of sand, even with one grain.'
Referred to as the "sorites paradox" or the "paradox of the heap", this argument is fallacious because there is no reason to suggest that a heap of sand is the same as one grain, just because there is a grey area between the two. There is still a difference, even if it isn't defined precisely. That is, there exists a qualitative difference, despite no precise quantitative difference.
- 'If it is allowable at birth for children with some grave abnormality, what will we say about an equally grave abnormality that is only detectable at three months? And another that is only detectable at six months? And another that is detectable at birth only slightly less serious? And another that is slightly less serious than that one?'
As in all instances of this form, the argument only holds if there is no point at which a distinction can be drawn between different points. One general distinction is that of voluntary and involuntary euthanasia, and it can be argued that this formation of the slippery slope does not hold, as there comes a point where a the child becomes able to express their own wishes. In abortion debates, a similar argument is proposed by arguing that there is no significant difference between a fetus at 14 days and a fetus at 15 days, or a fetus at three months and a fetus at three months and a day. This can be countered by pointing to significant changes in the development of the fetus that represent valid points of demarcation between stages of development.
 Countering the slippery slope argument
Frequently, the slippery slope has no evidence presented for why one would actually go down the slope, even if it is hypothetically possible. Pointing this out is sufficient to logically counter the argument. However, slippery slope arguments that involve no evidence can usually also slip the other way: if it is argued that gay marriage will lead to man-on dog sex, one could just as well argue that restricting gay marriage will lead to restrictions on other marriage, until it's banned altogether. After all, you may find it repugnant that two men marry, but if enough people find May-December romances repugnant, could we not also ban people from marrying anyone more than 4 years younger? If evidence is presented for why the slip could occur, the argument may be valid, in which case countering it requires refuting the evidence.
 See also
- Slippery Slope, Fallacy Files
- "A Misconception About Slippery Slope Arguments" at The Volokh Conspiracy
- "The Mechanisms of the Slippery Slope" (PDF), Volokh again.
- Your logical fallacy is slippery-slope, YLFI
- An X-Men comic which tells a powerful tale of anti-discrimination, but falls prey to the Slippery Slope
- ↑ A viewpoint on why something like this may not be so bad can be found here. WARNING: Fowl language.
- ↑ As if sex itself isn't obscene on its own, by those people's standards. Though they do tend to have a lot of kids.
- ↑ Glover, Jonathan (1977). Causing Death and Saving Lives. Penguin Books. ISBN 0-14-022003-8. p165