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Critically Thinking About the Slippery Slope "Fallacy"

Why is the Slippery Slope Argument perceived as fallacious?

The Slippery Slope Argument is an argument that concludes that if an action is taken, other negative consequences will follow. For example, “If event X were to occur, then event Y would (eventually) follow; thus, we cannot allow event X to happen.”

In recent times, the Slippery Slope Argument (SSA) has been identified as a commonly encountered form of fallacious reasoning. Though the SSA can be used as a method of persuasion, that doesn't necessarily mean it's fallacious. In fact, SSAs are often solid forms of reasoning. Much of it comes down to the context of the argument. For example, if the propositions that make up the SSA are emotionally loaded (e.g. fear-evoking), then it’s more likely to be fallacious. If it’s unbiased, void of emotion, and makes efforts to assess plausibility, then there’s a good chance that it’s a reasonable conjecture.

So, why is the SSA perceived as fallacious?

The SSA is perceived as fallacious primarily for reasons of relevance and certainty. With respect to relevance, what can make the SSA fallacious is that it may be used to avoid responding to propositions pertinent to the "here and now" of the argument at hand. It adds a component that isn’t necessarily relevant to the initial argument. For example, back in 2015, Ireland held a referendum regarding gay marriage, where citizens voted to legalise civil marriages between individuals of the same sex. One of the common arguments against the "yes vote" for same-sex marriage was that "if Ireland allowed gay marriage, then the next thing would be to allow same-sex couples to adopt." Regardless of how one felt about the "then" proposition, it was actually irrelevant to the propositions of the argument at hand. Whether or not adoption was legalised "down the road" had no bearing on whether or not two people of the same sex should be able to marry.

It is also worth noting that relevance, in this context, is also impacted upon, to a certain extent, by the acceptability or even desirability of the possible outcome. In the example above, it’s implied that the possibility of adoption by same-sex couples is a bad thing, which of course, not everyone believes. In fact, Ireland legalised adoption by same-sex couples soon after. As the SSA is a method of persuasion, it is by nature, biased — almost like a warning against an outcome the individual making the argument does not want to come to fruition. The interesting thing here is that the initial argument was fallacious in that it was presented in a negative light, much like a traditional SSA: “If event x were to occur, then event y would (eventually) follow; thus, we cannot allow event x to happen.” But, at the same time, the SSA was accurate in that "y" did eventually follow "x." Thus, it can be argued that the status of this argument as fallacious might just boil down to what individuals perceive as an acceptable outcome. However, the acceptability or desirability of the outcome shouldn’t really matter, given that what someone perceives doesn’t affect the likelihood of the conditions within the SSA from happening.

With respect to likelihood and certainty, the SSA is also often perceived as fallacious because it’s not possible for us to see into the future and guarantee that the subsequent event will occur. On the other hand, it’s also difficult to reject the claim for the same reason! But neither point necessarily makes this "if, then" conditional proposition unreasonable to suggest — it just requires assessment of plausibility. For example, as discussed previously, after critically thinking about patterns in human history, it may be possible to assess the plausibility of an event occurring in light of some previous event. If it’s plausible, then this SSA isn’t illogical.

To understand what's meant here by plausibility, it’s important to also understand that the foundation of the SSA is that it’s a conditional proposition, such as "if x, then y." Conditional propositions are commonplace in formal logic and are often logically sound. In some cases, arguments have multiple conditional claims which extend the "slope" — for example, "if x, then y" and "if y, then z." It might even go longer; for example, "if a, then b" and "if b, then c" and "if c, then d." However, in this context, the SSA is only fallacious if the propositions involved are false or implausible.

With that, the degree of plausibility, or even likelihood, of "x" leading to "z" or "a" leading to "d" cannot be assumed without careful consideration of the rest of the connecting conditional claims. If it’s plausible that "a" will eventually lead to "c," that implies that there is a good chance that both "a" will lead to "b" and that "b" will lead to "c." This is a reasonable way of considering conditional arguments, but it’s not entirely accurate because there is an underlying assumption that one will lead to the next and so on. The level of certainty requires thought.

When we consider such arguments in terms of likelihood, we see how we might be prone to overestimating the plausibility of the "event" occurring. For example, imagine that "a" leading to "b" is 90% likely, as are both "b" leading to "c" and "c" leading to "d." Many would be confident in the prospect that "a" would lead to "d" and may even give it a 90% chance of happening. But, this would be incorrect. What’s happening here is that the average across conditional propositions is being used. The reality is that the strength of the chain is weaker than the strength of each condition because it adds extra "if"s which affect the overall probability — thus, making "a" leading to "d" a 73% likelihood. Though 73% may be viewed as plausible, it’s still quite a bit less than 90%, so we need to be careful in our evaluation.

In conclusion, just because an SSA is used doesn’t mean that it’s wrong or fallacious. Sure, it’s commonly used as a method of persuasion; so, it requires evaluation. But, like other forms of argument, if it’s presented in a context that accounts for all available evidence, "for" and "against," so as to present the whole story, then it’s reasonable to consider its plausibility. On the other hand, SSAs can be wrong, unlikely, or emotionally loaded; but, so can other types of arguments. In reality, the only illogical course of action would be to dismiss an SSA without properly evaluating it first. As always, I’m very interested in hearing from any readers who may have any insight or suggestions, particularly perspectives from the field of argumentation!