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I sometimes hear someone claim that the person they are arguing with has "no evidence" for whatever they are arguing for.

Although I usually dismiss such claims thinking that what "no evidence" means is "no evidence I will accept", which is very different from "no evidence", I wonder if this might not be a named informal logical fallacy?

Recently it occurred to me that this claim of "no evidence" might be an example of the ad hominem informal fallacy. What the person is indirectly claiming by saying the other side has no evidence is that the other side is holding beliefs without any evidence and is therefore acting irrationally which sounds like an ad hominem attack to me.

But now I wonder whether I am assigning this fallacy correctly? Perhaps it is not a fallacy at all?

Hence the question: Is arguing that one's opponent has "no evidence" an example of some identified informal logical fallacy?

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  • 35 years ago you picked your nose. Therefore you can't be on the Supreme Court. Now, is it illegitimate for you to point out that I have no evidence? It hardly seems like a fallacy. On the contrary, if you note that I have no evidence, that's a pretty good point, right?
    – user4894
    Sep 18, 2018 at 17:29
  • I would say it is not an ad hominem attack because you are attacking the lack of evidence in the argument- not the strength of the argument because of the character of the person you are arguing against.
    – chevybow
    Sep 18, 2018 at 17:30
  • @user4894 I don't think it is always wrong to do this, but there is something suspicious about using that argument that might have been identified as fallacious reasoning in some contexts, that is, reasoning that a listener should handle with care.
    – Frank Hubeny
    Sep 18, 2018 at 17:32
  • @chevybow That's what I'm wondering as well. The ad hominem is indirect. It does more than imply there is no evidence, but that the opponent is also irrational.
    – Frank Hubeny
    Sep 18, 2018 at 17:34
  • @FrankHubeny Is attacking any argument automatically an ad hominem attack then- since it would claim the opponent is irrational for holding that argument?
    – chevybow
    Sep 18, 2018 at 17:55

2 Answers 2

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Hard to give a straight, unqualified answer here.

If you make a claim which you cannot back with evidence, there's no fallacy in your interlocutor's pointing out the fact.

If you make a claim which you can back with evidence (by standards pertinent to the subject) but which your interlocutor will not allow as evidence, your interlocutor is not perpetrating any kind of fallacy but may be cognitively incompetent.

Here's an example - unfortunately using the out-dated term, 'lunatic' :

A certain lunatic is convinced that all dons want to murder him. His friends introduce him to all the mildest and most respectable dons that they can find, and after each of them has retired, they say, 'You see, he doesn't really want to murder you; he spoke to you in a most cordial manner; surely you are convinced now?' But the lunatic replies 'Yes, but that was only his diabolical cunning; he's really plotting against me the whole time, like the rest of them; I know it I tell you'. However many kindly dons are produced, the reaction is still the same. (Henry E. Allison, 'Faith and Falsifiability', The Review of Metaphysics, Vol. 22, No. 3 (Mar., 1969), pp. 499-522 : 502.

No matter what, it will always be the case that 'you have no evidence' against the person's conviction - claim - that all dons want to murder him. But that is his problem, not yours.

If you make a claim which you can back with evidence (by standards pertinent to the subject) but which your interlocutor will not allow as evidence, your interlocutor may be perpetrating the fallacy of self-sealing argument. That's to say, his or her own argument or argumentative position may be vacuous in the sense that no evidence can defeat it : it is self-insulated from critique. It's the argument that's wrong here and not as in the previous case the arguer.

This might serve as an example (I'm not great on dialogue):

'I have a special fairy who always makes the best things happen to me.'

'But last year you broke your leg and this week your house burnt down !'

'Ah, but that must have been what was best for me, to save me from a worse outcomes.'

'How do you know ?'

'I have a special fairy who always makes the best things happen to me.'

Now, 'But last year you broke your leg and this week your house burnt down !', looks like pretty good evidence (not conclusive of course) that the special fairy does not exist. But it cannot break in on this self-sealed argument. Nothing can. Whatever evidence of sub-optimal outcomes you produce will always be deflected or deflectable by the 'explanation' that those outcomes were not sub-optimal because they happened in order to prevent something worse from happening. 'You have no evidence' that can break the seal.

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    I hadn't thought of something like "self-sealed argument". +1
    – Frank Hubeny
    Sep 18, 2018 at 20:00
  • Glad to oblige. Interesting question - several angles to figure out. Best : GT
    – Geoffrey Thomas ♦
    Sep 18, 2018 at 20:01
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    Wouldn't the 'lunatic' in the anecdote be guilty of the Real Scotsman fallacy? 'Aye, lad, but no real Scotsman eats pudding.'
    – user18050
    Sep 19, 2018 at 0:51
  • @Sean Boddy. I was uncertain just which heading to put the fallacy under. The Real Scotsman fallacy is self-sealing, I can see that, yes. They are both vacuous arguments at least. Well but, the reader now has your comment to add to my answer, and that's helpful. Thank you. Best - Geoffrey
    – Geoffrey Thomas ♦
    Sep 19, 2018 at 7:40
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While people often leave much of what they are thinking unsaid, and hence it is hard to call them out on a specific fallacy, I bet that quite a few people use "you have no evidence" in order to argue that what the other person is saying is wrong (and hence, as is so often the case, that they themselves, holding a contrary position, are right). If this is what is going on, then they are committing the fallacy of the appeal to ignorance.

Example:

"Smoking causes cancer"

"Oh yea? There is no evidence for that. Smoking is perfectly safe!"

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  • That scenario about smoking would be an example of what I'm thinking of. +1
    – Frank Hubeny
    Sep 18, 2018 at 23:50
  • You're into a whole new category of dishonesty with this particular argument. The argument about smoking has become dogma, and I can demonstrate it. Lab experiments are the basis of 'proof' that smoking is bad for you, because it's nigh on impossible to set up large scale control groups. And given that many smokers smoke 20 a day for 40 years, what exactly is the definition of 'harmful'? Try drinking 20 mouthfuls of Coca Cola a day and see how you fare. I suggest the anti-smoking lobby dogmatic useful idiots, the government wants to kill old people, and smoking extends you life in fact.
    – Richard
    Sep 19, 2018 at 13:14

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