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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.
 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. (I.e., Santorum's "man-on-dog sex" statement listed above) 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.
This is 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.'
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.
 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 banning marriage altogether. 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