The Slippery Slope

Source: Smithsonian Magazine

Some names for informal logical fallacies are relatively easy to remember because they're associated with particular images. The slippery slope is one of the easiest informal fallacies to remember because of its graphic name and because we often see examples of the slippery slope fallacy in everyday life. Again, we often see this fallacy in politics.

But the slippery slope fallacy can also be difficult at times to spot for a couple of reasons. First, it can be difficult to spot the instances of the fallacy if the fallacy is being used to draw a conclusion that we agree with. Second, the slippery slope is often confused with a form of argumentation that is valid. If you've read my introductory post on logic (I encourage you to do so), then you know that there are rules by which a simple deductive argument can be considered valid (i.e., the conclusion follows necessarily from the premises). One of those valid logical rules is called modus ponens and goes like this:

  1. p -> q

  2. p -----------------

  3. Therefore, q

* Where p and q denote discrete propositions, and -> denotes an implication. In other words, read p -> q as "If p, then q."

This rule allows you to draw a novel conclusion, for which you may not have much evidence, from another proposition whose truth implies the truth of the conclusion and for which you have sufficient evidence for believing. Another important rule of deductive logic is hypothetical syllogism, and it goes like this:

  1. p -> q

  2. q -> r -----------------

  3. Therefore, p -> r

The hypothetical syllogism extends the logic of modus ponens to further conclusions that are implied by the conclusion of the original inference. This reasoning can be extended further than simply two premises (the steps above the line) and one conclusion (the step below the line). For instance, consider the following argument:

  1. p -> q

  2. q -> r

  3. r -> s

  4. s -> t

  5. p -----------------

  6. Therefore, p -> t (HS)

  7. Therefore, t (MP)

* The abbreviations in parenthesis are the rules used to draw the conclusion. So, "HS" means "hypothetical syllogism," and "MP" means "modus ponens."

Technically speaking, as the argument is written, I skipped some steps, but this was intentional. According to HS, p directly implies t by implying one propositions that implies another, and then another, and so on. Because of this, it was unnecessary to write under the conclusions "Therefore, q," and "Therefore, r," and so on. There is, however, nothing incorrect in writing out all of the conclusions. Because of the combination of the HS and MP rules, a single true proposition can be used to justify a seemingly distant conclusion. It is this combination in one's deductive reasoning that is often called the slippery slope fallacy.

So, what exactly is the slippery slope fallacy? As an informal logical fallacy, the slippery slope doesn't always appear in the same way and for the same reason. Try to think of a slippery slope in terms of the links in a chain. If one is holding a length of a chain, one can move one's hand down the chain, link by link. Let's say that the chain one is holding has links that are marked in succession by a letter. The first link is marked as "A," the second as "B," the third as "C," and so on. Let's say that there are ten links in this particular chain, such that the last link of the chain is marked with the letter "J." In this illustration, it is easy to see that each of the links are integrally connected to one another. Link A is not directly connected to link D but they are connected via A's connection to B and B's connection to C. This is a good way to imagine an argument based on HS and MP.

The slippery slope fallacy would not say that A implies J. In our illustration, such an argument would be valid because of HS and MP. The slippery slope would be like claiming that A implies P, where "P" is a link completely disconnected from the whole chain. In other words, if a chain of reasoning from a premise to a distant conclusion is not based on a chain of inferences according to HS and MP, then that chain of reasoning is a slippery slope. If the argument in question uses HS and MP, then it is not a slippery slope, even if the conclusion seems distant from the premise.

What are some examples of slippery slopes? As I've already indicated, apparent slippery slopes are common in politics on both sides of the political spectrum. For example, those on the Left in the United States have argued recently that if the Supreme Court were to overturn Roe v. Wade, this would open the door for overturning Obergefell v. Hodges (same-sex marriage), Loving v. Virginia (interracial marriage), and Griswold v. Connecticut (contraceptive access without government restriction). On the Right in the United States, it has been commonplace for sometime to argue that the acceptive and normalization of sexual practices outside Judeo-Christian norm would justify all kinds of sexual deviancy, including pedophilia and bestiality. Any one of these arguments might be examples of the slippery slope, and I'm not necessarily claiming that either of the above arguments is fallacious.

The key is that, if the argument is not to be fallacious, the connection between the initial premise and the eventual conclusion must be made explicit. If it isn't explicit, then we have no reason not to assume that the argument is just a slippery slope. This is helpful for at least three reasons. First, it requires those who would make these kinds of arguments to defend the supposed inference, as the inference is usually both shocking and far removed from the initial premise. Second, as the one who must consider the value of these arguments, it allows us to be skeptical of their validity unless shown that the inference is valid. The burden of proof to show that the argument is valid is on those who would make these arguments. Finally, it challenges us not to immediately accept arguments along these lines that sound good to us. Some of my readers will be from either side of these political issues. Don't just latch onto the nearest argument because it aligns with your presuppositions. Instead, be careful to rationally analyze these arguments for yourself and be willing to point out fallacies, even from your side of these debates.

That's it for this post! These posts are intended to very short and easy to read. I want them to a helpful resource for reference and an introductory resource for people who haven't been exposed to these concepts. I hope that the content will be edifying to you. If you have questions or would like to reach out to me to discuss these things, feel free to comment here or to send me an email or message on Facebook. If this blog has been helpful and interesting for you, feel free to subscribe to be notified of any new posts. Finally, if this post would benefit anyone else, please feel free to share it on social media. Thanks for reading!


For the sake of full disclosure, this is the main resource that I will be using for this series. This is an excellent textbook and worth checking into if you want more information on the subject:

Hurley, Patrick J. A Concise Introduction to Logic. Twelfth Edition. Stamford, CT: Cengage Learning, 2015.

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