Is there a fallacy which claims

  1. Mr Smith claims X
  2. Mr Smith is clearly irrational about some things
  3. X isn't true.

I suppose a kind of appeal to not being an authority - but I mean specifically an appeal that the speaker isn't an authority in something else entirey.


gnasher729 correctly points out that the second premise of this argument needs modification if the argument is going to be at all plausible. But even if we swap 'some' for 'all' in the second premise the argument is still invalid.

The (modified) argument is invalid (i.e. is fallacious) iff the premises can be true and the conclusion false. Here's a scenario where the premises are true and conclusion false: Smith claims (and, presumably, believes) that the sky is blue. But Smith believes that the sky is blue because he consulted an 8-ball (I take it that this is not in general a rational way to go about determining one's beliefs) - moreover this is how Smith goes about deciding all of his beliefs. So Smith is irrational. But the sky is in fact blue. So both premises are true, and the conclusion false. So the argument is invalid, hence fallacious.

Here's a deeper reason why the argument doesn't work. What it's rational to (dis)believe doesn't neatly line up with what is true (respectively, false). There are instances where it is rational to believe a falsehood (suppose you're a physicist in the late 1700s who accepts Newtonian physics). And there are instances where it's irrational to believe what's true (as in Smith's case). So what it is (ir)rational to believe isn't a universally reliable guide to what is (un)true. Which isn't to say that all of our rational beliefs are false; likely many of them are true. But it is to say that rationality isn't a reliable criterion for truth. (Very roughly - many philosophers would disagree with what I've said here. 1980's Putnam comes to mind).

  • +1 but with a slight hesitation about calling Newtown physics a "falsehood."
    – virmaior
    Aug 2 '15 at 3:03
  • @virmaior - Rereading my post I agree it's a little weird to call Newtonian physics a falsehood. Was the hesitation for you because falsehood is a property of propositions, and Newtonian physics isn't a proposition? Or because Newtonian physics isn't false? Aug 2 '15 at 14:06
  • I am not sure I fully grasp the distinction between the two cases (or at least if we don't view theories as false, then we can't view this one as false either). I guess on the simplest level I wonder how helpful it is to speak as if only 18th Century physicists are not irrational for accepting Newtonian physics. It seems perfectly rational to keep using Newtonian equations even in the 21st Century to solve a large number of physics problems, and practically irrational to force QM onto these problems.
    – virmaior
    Aug 2 '15 at 23:32
  • That's fair, and I'm sympathetic to what you're saying. I don't think anything in the OP really hinges on talking about Newtonian physics - I was just using it as a placeholder for some false theory. Maybe something about phlogiston would be better. Aug 3 '15 at 3:44

There are two fallacies here at the same time.

First, taking the premise "Mr Smith is clearly irrational about some things" and concluding "Mr Smith is irrational about everything".

Second, taking the premise "Mr Smith is irrational about something" and concluding "Mr Smith's claims about something must be wrong". If Mr Smith is irrational about something, then his opinion will be totally unrelated to the facts, not the opposite of the facts.

(Plus the minor detail that Mr Smith might be trying to mislead you, and what he claims might be the opposite of what he believes. And since he is sometimes irrational, that's not too unlikely).

  • Smith can be irrational and still have all true beliefs - suppose he just adopts beliefs at random and happens to get lucky every time. It's not that his irrationality means his opinions are unrelated to the facts, it's that his justifications for his beliefs don't live up to the standards required to be rational - flipping coins (or whatever) just isn't a rational way to go about deciding what one should believe. Aug 1 '15 at 23:59

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