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, e.g. 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.
While the slippery slope may in fact exist, usually the argument is put forth while ignoring possible mitigating factors (such as informed consent for sex partners in the Santorum example), thus making it generally a somewhat-too-serious form of reductio ad absurdum.
 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 accredited to former U.S. Supreme Court Justice William O. Douglas.
 Example of the Slippery Slope Argument
Logical fallacies are a dangerous thing!
And after that, you're well on your way to hosting a show on Fox News!
Avoid logical fallacies at all costs; there's no telling where they'll lead!
 Countering the slippery slope argument
Frequently, the slippery slope argument 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.
- ↑ 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.
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