Slippery Slope

Description:

The argument tries to defend a status quo by appealing to the falsity (or undesirability) of some presumed inevitable consequence of change.

Examples:

"We can't allow terminal patients to die without doing everything to save them. If we were to allow that, next we would be permitting euthanasia, and then we would start killing people outright whenever we thought they were no longer valuable to society."

"Laws prohibiting the possession of handguns are the first step toward robbing us of all our civil liberties!"

"Obamacare is an attack on our liberties, and the first step in a socialist conspiracy!"

“A lot of the people that put me where I am are strong believers in the Second Amendment, and I am, also. And we have to be very careful about that. You know, they call it the ‘slippery slope,’ and all of a sudden, everything gets taken away. We’re not going to let that happen.” - Donald Trump, August 20, 2019, at a press conference.

Discussion:

In a reductio ad absurdum argument, the arguer assumes (for the sake of argument) that the opponent's position is true, and then shows that the consequences of this assumption, taken to its logical conclusion, is unacceptable. For example, to prove the falsity of the claim, "Ralph can do anything," assume that Ralph can do anything. Then think of something for Ralph to do that would limit Ralph's ability to do something else. Ralph might make a wall so high that no one could jump over it. But if no one can jump over it, then Ralph cannot jump over it, so (since Ralph can do anything) it follows that Ralph both can and cannot jump over the wall. But this is absurd; so the initial assumption that Ralph can do anything must be false.

Reductio ad absurdum is a legitimate form of refutation. Statements really do have logical consequences, and where those consequences are unacceptable, those statements must be unacceptable as well.

The Slippery Slope fallacy mimics the pattern of the reductio ad absurdum argument. It postulates the truth of an opponent's position, and then tries to make the case that the opponent's position would lead to unacceptable consequences. However, the Slippery Slope fallacy draws consequences that are not actually logical consequences of the opponent's position. Rather, the opponent's position is "connected" to the unacceptable consequences by some other means, usually some sequence of events that might conceivably follow, but are not guaranteed to do so, and indeed probably would not. The childrens' author Laura Numeroff explores this kind of slippery slope argument in If You Give a Mouse a Cookie, and its many delightful sequels. Other times the argument turns on a psychological continuum, i.e. that we will slowly become accustomed to things that we currently find unacceptable. Such psychological continuums do exist, but movement is rarely only in a single direction, so movement to an unacceptable extreme is never inevitable.

Source: I first became aware of this fallacy from Howard Kahane, Logic and Contemporary Rhetoric (1971). Although this is almost certainly not the earliest reference to this fallacy, I have not so far been able to identify an earlier source.

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