argumentum ad fidentia
(also known as: against self-confidence)
Description: Attacking the person’s self-confidence in place of the argument or the evidence.
Person 1 claims that Y is true, but is person 1 really sure about that?
Therefore, Y is false.
Rick: I had a dream last night that I won the lottery! I have $1000 saved up, so I am buying 1000 tickets!
Vici: You know, dreams are not accurate ways to predict the future; they are simply the result of random neurons firing.
Rick: The last time I checked, you are no neurologist or psychologist, so how sure are you that I am not seeing the future?
Vici: It’s possible you can be seeing the future, I guess.
Explanation: Although Vici is trying to reason with his friend, Rick attempts to weaken Vici’s argument by making Vici more unsure of his position. This is a fallacious tactic by Rick, and if Vici falls for it, fallacious reasoning on his part.
Chris: You claim that you don’t believe in the spirit world that is all around us, with spirits coming in and out of us all the time. How can you be sure this is not the case? Are you 100% certain?
Joe: Of course not, how can I be?
Chris: Exactly! One point for me!
Explanation: This is a common fallacy among those who argue for the supernatural or anything else not falsifiable. If Joe was not that reasonable of a thinker, then he might start to question the validity of his position, not based on any new counter evidence presented, but a direct attack on his self-confidence. Fortunately for Joe, he holds no dogmatic beliefs and is perfectly aware of the difference between possibilities and probabilities (see also appeal to possibility).
Exception: When one claims certainty for something where certainty is unknowable, it is your duty to point it out.
Tip: Have confidence that you are probably or even very probably right, but avoid dogmatic certainty at all costs in areas where certainty is unknowable.
This a logical fallacy frequently used on the Internet. No academic sources could be found.