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I've recently come across a particular errant pattern of argument a couple of times, and I'm wondering if there is a name for this fallacy.

The form of the argument is:

  1. A has property X
  2. B has property X
  3. Therefore A and B are the same thing.
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    I am surrounded by this too, generally in the form of conspiracy theories about Big X. Big Pharma's profit motive is to encourage a large number of consumers of drugs. The spreading of diseases encourages a large number of consumers of drugs. So Big Pharma's profit motive is best met by the spreading of diseases. Ergo, they are motivated to encourage us all to have impaired immunity. (And so they are in bed with Big Processed Food to make sure we are well-fed but malnourished, which is why they don't want us to eat marijuana...)
    – user9166
    May 3, 2015 at 17:26
  • Do you have a more specific example? If X is the defining property of A, then this might not be a fallacy.
    – Memming
    May 4, 2015 at 19:16
  • @Memming, the specific case would be overly controversial to put here. But I'd say with utmost confidence that X wasn't a defining property. May 5, 2015 at 2:36
  • Looks like it's closely related to the scientific axiom, correlation is not causation. May 10, 2015 at 15:09

2 Answers 2

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I would call this form a faulty argument by analogy. Some links on the internet will claim that this is the fallacy argument by analogy, but I think the category is not automatically invalid.

What makes it faulty or not is the extent to which the property in question makes two things the same (presumably the same category rather than the same physical object).

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Descarte used a form of this reasoning; he suggested since matter has extension and space is extensive then they are in essence the same; this was picked up by Spinoza where he said the essential attribute of the physical world was extension.

So, though it might be a fallacy to equate them; it might be pointing to something deeper.

Here is another example from physics: one of the most established results in quantum gravity is Bekenstein-Hawking radiation; this came about originally by an observation of Bekenstein that since the event horizon of a black hole could only increase and since in general the entropy of a system could only increase too; then we should identify them; the obvious counter-answer is that this means that black holes have temperature; which means that they radiate; but classically they don't - so a contradiction never mind it being a fallacy; it turned out though when Hawking set out to prove Bekenstein wrong he found his intuition held up.

So perhaps this fallacy is not always a fallacy...and is it fallacious to call a fallacy a fallacy if it sometimes fails at being a fallacy?

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