Wednesday, May 26, 2010

Pink Rabbits

The 'experts' at Britannica blog commit another howler... here's my response (awaiting moderation).

Goodness, if you’re going to attempt to use logic to make your point, at least avoid uncharitably confusing an inductive inference with a categorical syllogism.

Sullivan is obviously making an inductive inference. He is inferring from, “I have seen thousands of gang members and none of them have been incarcerated for possession” to “gang members are not generally incarcerated for possession.”

It’s the same form as “I have seen thousands of rabbits and none of them were pink, thus, rabbits are not pink.” It does *not* assume that “Only those rabbits which I have personally witnessed actually exist.”

The appeal to an implicit premise (it’s called a ’sorite’) is common and can, indeed, lead to fallacies (see ‘The sorites paradox’ ). This is not one of them. This is pure and simple logical blundering, or what we would call in philosophical circles, a howler.

For more on logical fallacies (real logical fallacies, not the fake one described here) you can see my


  1. Bob McHenry Says:
    I’m sorry, Mr. Downes, but I think you are wrong on several counts.
    1. If you look more carefully, you’ll see that it is not Mr. Sullivan but one of his readers who is making the argument.
    2. His conclusion is not that “gang member are not generally [my emphasis] incarcerated” but that “people in the U.S. ARE NOT incarcerated.” Such a categorical conclusion points to syllogistic reasoning, not inference.
    3. The appeal to an implicit premise is called, as I wrote, an enthymeme.
    4. A sorites (ending in -s, though not a plural) is, as Britannica explains, “in syllogistic, or traditional, logic, a chain of successive syllogisms,” or, as it is sometimes called, a polysyllogism.

  2. Stephen Downes Says: Your comment is awaiting moderation.
    You’re right about ‘enthymemes’ - I couldn’t bring the right term to mind.
    But you’re simply wrong about the argument.
    Interpreting that argument as anything other than an inductive argument is a travesty. It doesn’t matter how he worded the conclusion. Inductive arguments are frequently concluded with categorical conclusions. And *nobody* would suppose that “Only those things which I have personally witnessed have actually happened.”

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