What Not to Do: If people are committed to defending a specific issue, don’t bother trying to persuade them of the inevitable ill consequences and outcomes by appealing to a slippery slope (i.e., an idea or course of action that will lead to something unacceptable, wrong, or disastrous).
Why Not to Do It: The problem with slippery slope arguments is not that they aren’t legitimate or valid (they certainly can be). The problem is they’re generally rhetorically ineffective because many people either fail to think logically or are apathetic about the resulting consequences.
A slippery slope argument is merely a claim that “A will lead to B” either as an inevitability, as an increased probability, or as a logical outcome. Slippery slope arguments are often misunderstood, and many people mistakenly think their use is always logically fallacious. As a general rule, if someone summarily dismisses a slippery slope claim, he or she is probably not the type of person who understands how arguments work. (For instance, there are numerous legal precedents where people have argued that accepting legal decision A will lead to B and have been proven correct.)
A full defense of slippery slopes against supporters of folk fallacies will have to wait for another day. For now, I’ll simply refer to and recommend one of the best analyses and explanations of the slippery slope, Eugene Volokh’s 2003 article in the Harvard Law Review, “The Mechanisms of the Slippery Slope.” In his paper Volokh says,
A slippery slope is one that covers all situations where decision A, which you might find appealing, ends up materially increasing the probability that others will bring about decision B, which you oppose.
If you are faced with the pragmatic question “Does it make sense for me to support A, given that it might lead others to support B?,” you should consider all the mechanisms through which A might lead to B, whether they are logical or psychological, judicial or legislative, gradual or sudden.
You should consider these mechanisms whether or not you think that A and B are on a continuum where B is in some sense more of A, a condition that would in any event be hard to define precisely.
In order to take a slippery slope argument seriously, support for position A needs to lead to the realistic possibility that people will support position B. Absurd scenarios can be dismissed if they are truly absurd. For example, if someone claims that if we let religious people opt out of the contraceptive mandate we’ll have new religious groups (e.g., The Church of Anti-Contraception) springing up in order to get out of paying for healthcare services, they are not making a legitimate slippery slope argument—they’re just spouting rhetorical nonsense.
So if slippery slope arguments can be valid, why shouldn’t we use them? The main reason is that if you need to point out a slippery slope it’s likely to not persuade the person you are arguing against.
A primary appeal of slippery slope arguments is that they are almost always obvious. For example, if Argument A is that “People should be able to marry any other adult” then that supports Argument B: “Adult siblings should be able to marry each other.” Most people would agree that it’s obvious A leads to B, so the person making Argument A needs to modify or change his position. However, the fact that it wasn’t obvious to the supporter of Argument A means you will likely waste a lot of time trying to get him to notice what should be easily perceived.
More often, though, the reason people reject slippery slope arguments is that they support A even if it leads to B—they just don’t want to admit it yet. A prime example is with homosexual marriage. The arguments used to support that radical change also can be used to support expanding the definition of marriage to include incestuous and polygamous relationships.
Logically speaking, this was never really disputable. But many advocates of homosexual unions did dispute it because it weakened their own position. Now that their own position is gaining public acceptance, though, some are willing to now shrug and say, “So what if it does lead to polygamy?”
Once we’ve slidden down the slippery slope, they are usually more willing to admit that their position does support B, and they just aren’t all that concerned about it. For people who were concerned, though, such an admission comes too late.
What to Do Instead: Appeals to a slippery slope are a form of shorthand. To persuade those committed to a position, though, usually requires a more long-form presentation that “connects the dots” showing “A leads to B.”
One way to do that is to clarify the unstated warrant. A warrant is the chain of reasoning that connects the reason to the claim. For example, “Needle exchange programs should be abolished [claim] because they only cause more people to use drugs.” [reason].
The unstated warrant is: “when you make risky behavior safer you encourage more people to engage in it.”
Unless you get agreement (or at least a concession) that the unstated warrant is true, then you are not likely going to convince the person who accepts position A.
However, you aren’t likely convince him anyway. Your true audience is those who are not yet convinced and are open-minded enough to still consider the logical outcomes. Helping them to see the unstated warrant and how it connects A to B can keep them from tripping up on bad arguments and finding themselves at the bottom of an unfortunate rhetorical slope.