I am trying to get to grips with an argument with a friend, whereby he argues that because he does not have the full facts or because one theory is not 100% proven means that both competing theories must be considered valid.

Note that this is used in the case of the earth being 6000 vs billions of years old or the moon landings were faked or not.

A precise of his argument being that because he has not looked into the whole whether the moon landings were faked or not thing, he can validly claim to not know if we landed on the moon or not.

So is this "I cannot know, therefore I can stay on the fence" argument a form of the argument ad ignoratum logical fallacy?

  • This question might also be related to and help you think about your argument with your friend: philosophy.stackexchange.com/questions/14262/… ("if [...] the idea that anything we believe we know, at any point in history, could be wrong, why should we continue pursuing science?")
    – Dave B
    Nov 29, 2018 at 21:24

4 Answers 4


The argument from ignorance fallacy illegitimately takes lack of evidence against something as evidence for it, so this does not actually qualify.

Perhaps your friend, annoying as it may be, simply has very high standards for what he considers as "knowledge". It might be worth investigating to see if he thinks he "knows" anything. If he does claim to know anything, you could then test whether or not his standards for knowledge are consistent. If he doesn't claim to know anything, however, he's just following in the footsteps of Socrates (whom people also found incredibly annoying).

With all that said, if your friend is actually arguing that (for instance) the arguments for belief in the moon landing are equally as strong as the arguments against it (and those are his words, not yours) then that is in itself a knowledge claim. If his argument for that claim is that he does not know enough to judge which argument is stronger, then that is in fact an argument from ignorance, since he would in that case be arguing from his own lack of evidence to the illegitimate claim that both arguments are equal in power (rather than simply acknowledging his own lack of ability to judge them).

  • 1
    At least I know now why they wanted Socrates to eat hemlock...
    – Nat
    Dec 5, 2013 at 23:19

This reminds me of a joke that the probability of every event is 50%: there are exactly 2 possible outcomes, the event either happens or does not happen.

Jokes aside, just as Chris pointed out, you are confusing the refusal to make a statement about the validity of a proposition with making a statement that the probability of the proposition is exactly 50%. Just because you don't know whether the proposition is valid doesn't make the probabilities of the alternatives equally valid.

For example, suppose I draw a random card from a 52-card deck and allege that it's Ace of Spades without even looking. You don't know the card either, and you cannot be 100% sure that I am mistaken. However, your lack of knowledge does not imply that the probability that I drew the Ace of Spades is 50%.

The situation with most conspiracy theories is somewhat like that: they make a claim, validity of which cannot be easily determined, although the probability of its validity is microscopic. Then they erroneously claim that the validity of the claim is 50% simply because there are exactly 2 alternatives: either it's true or not.

  • 1
    Related: en.wikipedia.org/wiki/Principle_of_indifference
    – user3164
    Dec 5, 2013 at 19:52
  • 1
    A related notion is that lying does not require affirmative knowledge that a claim is actually false. Someone who claims that "The Foo is Wizzle" is really saying "I have knowledge that shows that the Foo is Wizzle"; if the person has no such knowledge, the statement is a lie, even if the Foo happens to be Wizzle. For example, someone who draws two random cards and claims without looking that they are the Spade Ace and Diamond Jack is lying. There's a roughly-one-in-1300 chance that they did in fact draw those cards, but they have no basis for claiming that they know that.
    – supercat
    May 13, 2014 at 13:17

Without the statement of a purpose or telos, you don't have a standard with which you can judge your friend's position. Consider the following two purposes:

  1. I want to increasingly understand how reality works.
  2. I want to feel that I know things.

These might seem very similar, but there is a crucial difference. #2 allows for 'just-so stories', stories that allegedly explain something but offer little to no promise of further expanding our knowledge. #1 speaks of a tradition which can have life—where knowledge begets knowledge—or death, where stagnancy reigns. Some like stagnancy, preferring to call it 'security' or 'comfort' or something like that.

Those who engage in #1 necessarily must operate in the presence of probabilities: maybe the phenomenon happens for this reason and maybe for that reason. The key is to be able to figure out which reason is more likely to be the case. The word 'valid' becomes much less than your friend is making it out to be (I think he is implying 'equally valid'): it just means that a given hypothesis is sufficiently likely to be useful to not be discarded.

Perhaps your friend only wishes to do #2. If so, then perhaps there is nothing 'wrong' with his current approach. I don't know whether there is a named logical fallacy going on here; I think we need to know more about what your friend wants before looking for fallacies. Not all people want the same things.

  • Nice addition. My mate is not actually going for "equally valid", but more along the lines of "I have no facts to evaluate the arguments therefore both are equally unknowable". A subtle distinction, but he is trying to be annoying or "truthful" as he calls it, he holds no belief as such. I guess if we were arguing something such as global warming caused by nature vs man his position would be more understandable.
    – Nat
    Dec 5, 2013 at 23:16
  • @Nat: perhaps you would like looking at prior probability, which talks about what we should think before we have any evidence. I think one must choose a prior based on a purpose, for when we say that A is 'better than' B, we almost always measure by whether A accomplishes some purpose better than B. Hypothetical imperatives are made 'binding' by purpose. The attempt to find a categorical imperative of any sort has not met with what many consider 'success'. :-)
    – labreuer
    Dec 5, 2013 at 23:50

Here is the question:

A precise of his argument being that because he has not looked into the whole whether the moon landings were faked or not thing, he can validly claim to not know if we landed on the moon or not.

So is this "I cannot know, therefore I can stay on the fence" argument a form of the argument ad ignoratum logical fallacy?

Consider Douglas Walton's description of this argument's form (page 2, taken from David Kelley, The art of reasoning):

¬A has not been proven true.

Therefore, A is true.

Your friend does not appear to be making an argument for one side or the other. Rather your friend is staying on the fence. Perhaps your friend is feeling social pressure from people with differing beliefs and your friend does not want to offend any of them.

Based on Walton's proposed description of the form of this argument style, since your friend is not claiming that one of those sides is true, your friend could not be making an argument from ignorance.


Walton, D. (2010). Arguments from ignorance. Penn State Press.

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