5X5 Episode 101

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5X5 Episode 101
Slippery Slope/Post Hoc
26th January 2011

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5X5 100 5X5 102
Skeptical Rogues
S: Steven Novella
R: Rebecca Watson
B: Bob Novella
J: Jay Novella
E: Evan Bernstein
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Skepticism 101 - Slippery Slope/Post Hoc[edit]

You're listening to the Skeptics' Guide 5x5, five minutes with five skeptics, with Steve, Jay, Rebecca, Bob and Evan.


S: This is the SGU 5x5 and tonight we're talking about a pair of logical fallacies. This is part of our ongoing series on Skepticism 101; some basic concepts that are useful for every skeptic. We've gone over a number of logical fallacies before. A logical fallacy is essentially when the logical connection between the premise and the conclusion does not follow; there is some problem with the logic. We're going to talk about "post hoc ergo propter hoc", but first we're going to talk about the logical fallacy of the slippery slope. Essentially, this is the logical fallacy that involves—assuming that taking one step or accepting one occurrence or one outcome necessarily leads to a far more extreme version of that outcome.

E: Yeah, and there are actually two types of fallacies referred to as slippery slopes. One type is known as the "causal" version, or in Latin "non causa pro causa", which claims that "A" leads to "B", "B" leads to "C", "C" leads to "D", and so on until one finally claims that "A" leads to "Z". Now, while this is formally valid when the premises are taken as a given, each of those steps or each of those contingencies needs to be factually established before the relevant conclusion can be drawn. The causal version of the slippery slope fallacy occurs when this is not done. For example, if I were to say that if government passes laws against fully automatic weapons, then it won't be long before government passes laws on all weapons, which will then lead to the restriction of other rights, and finally we end up living in a totalitarian state. Thus we should not ban fully automatic weapons. This is a classic causal slippery slope fallacy.

B: The "semantic" version of the slippery slope fallacy is related but it is distinct in its own right. The idea here is that the top of the slope and the bottom of the slope, if you will, are different due to a series of tiny insignificant changes between them or differences between them. Since there is no obvious point anywhere that clearly distinguishes the top from the bottom, there's therefore no real difference between the two, or possibly, this could also mean that the top or bottom doesn't exist at all. In the semantic slippery slope argument then, the argument is that two things may seem different but are really the same, since the difference between them is arbitrary. Another way to look at this fallacy is to consider "when is a beard a beard?" How many hairs does a beard have if it's more than one?

S: Another example might be the difference between tall and short. When does a short person become a tall person; at what height? There is no objective unambiguous dividing line, therefore you deny both extremes, but that doesn't mean that, in fact, there isn't some meaning to the concept of tallness or shortness, as long as it is understood as ends of a continuum.

R: And that brings us to our next logical fallacy, "post hoc ergo propter hoc". Basically, the idea of this is that it's sorta similar to what we often say on the Skeptics' Guide, which is "correlation does not equal causation". Basically, things that come after one another aren't necessarily causally linked. So we often talk about the autism and vaccine debate. A lot of parents think that when their kids get a vaccine and then later develop autism they assume that the two things must be related because the autism follows the vaccine. However, the science shows us that those two things are not related and that's exactly what the translation of "post hoc ergo propter hoc" is: "after this therefore because of this". It's not necessarily true and therefore it is a fallacy.

J: A good example of the post hoc logical fallacy would be taking vitamin C after you feel the onset of a cold. A lot of people take vitamin C when they get sick and they believe that the fact that they feel better faster than they previously thought they would means that the vitamin C helped them feel better. But that is an anecdotal experience, meaning one person taking vitamin C at home without being involved in an extensive study with, you know, hundreds or maybe thousands of people involved, doesn't really conclude anything other than they got better from that particular cold in x amount of time.

S: And with both of these logical fallacies, it's important to recognize that the fallacy is in assuming that these conclusions necessarily follow. That if "B" follows "A" then "A" must have caused "B" or that that is sufficient in order to conclude that "A" caused "B". When, in fact, that is not necessarily so. Or that, you know, banning assault rifles will lead to a police state. That, in fact, may happen but it's not necessarily true; the logic itself does not follow. And that's what makes these two examples of logical fallacies.

S: SGU 5x5 is a companion podcast to the Skeptics' Guide to the Universe, a weekly science podcast brought to you by the New England Skeptical Society in association with skepchick.org. For more information on this and other episodes, visit our website at www.theskepticsguide.org. Music is provided by Jake Wilson.


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