A slippery slope argument is one that warns against a particular action, not so much on the grounds that that action is bad in and of itself, but because it “opens the door” to a chain of events leading downward to a bad outcome.
Or, “The problem with A is if you let that happen, then that’ll lead to B, and once you have B it’ll be impossible to avoid C, which will inevitably lead to D, and before you know it, E will happen and then you’ll be sorry.”
An example of a slippery slope argument would be if I were to oppose a proposal in my state to require handgun purchasers to pass a gun safety class because “If this passes, then pretty soon it’ll be all guns rather than just handguns. And then it’ll be not just a safety class, but psychological profiling to ensure you don’t have ‘anger management issues’ or political opinions the government doesn’t like. And don’t think they’ll stop at new purchasers; next it’ll be people who already own a gun who have to pass these ‘tests.’ Then they’ll come up with more and more hoops to jump through to make it harder and harder to own a gun legally. Eventually they’ll just drop the pretense and confiscate all guns from private hands.”
So my argument wouldn’t be that certifying that people who buy handguns have some knowledge of gun safety is bad taken by itself, but that it’s bad in that it’ll lead to confiscation of all guns.
Another example would be if I say it’s important not to allow students in a public school to take turns leading their class in a voluntary group prayer at the start of the school day, because “If you let this happen, then soon it won’t be a student leading the prayer, but the teacher leading a government-approved prayer. Then they’ll make it so it’s not so ‘voluntary’ any more, that all students will be required to participate if they choose to remain in public school. And it won’t stop at prayers; next the curriculum itself will be altered to fit the religious preferences of the majority. Once the public schools have been transformed into religious schools, other aspects of society will be gradually turned over to religious authorities until we’re living in a theocracy.”
Again, I wouldn’t be saying that a group of students praying together as a voluntary group for a couple minutes during the school day in and of itself would do some great harm, but that we need to be wary of it because otherwise we risk ending up in an oppressive theocracy.
Are slippery slope arguments always fallacious? The answer-as is true for most fallacy types-is no. Sometimes reasoning in this fashion constitutes a fallacy; sometimes it does not.
Think of a slippery slope argument as a chain of causal claims or conditional predictions: A will cause B (or make B more likely, or embolden the bad guys who want B to try for it and achieve it, etc.), B will cause C, C will cause D, and so on down the line. Causal claims, or predictive claims about human behavior, are not somehow necessarily fallacious. Some are justified to varying degrees by the available evidence, and some have little or no such justification. Each needs to be judged on its merits.
But if each link in the chain of a slippery slope argument is justified, then the argument as a whole might well be. So just as an individual causal claim should be examined on a case by case basis and should stand or fall on its merits, so should a slippery slope argument.
It’s worth noting, though, that as a rule of thumb slippery slope arguments tend to be fairly weak. This is because they are so often offered in areas like public policy, where we’re talking about long term effects on group behavior. It is treacherous to make even fairly simple predictions in areas like economics, politics, and foreign and military affairs, so when someone alleges a whole chain of such causal predictions, the uncertainty factor soon multiplies out of control.
Still, we need to do the best we can at making projections about the future. One would be a fool to close one’s eyes to the long term and indirect consequences of today’s decisions just because the future is hard to predict.
We need to be responsible about even the long term consequences of our actions, while not falling prey to evidence-free, slippery slope “warnings” that some otherwise innocuous step we take today will send us inevitably careening toward Hell in a hand basket.