What Is This Fallacy?
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Original Question
Person B asks for very specific proof from the past, and when Person A cannot, B claims A is lying.
Example:
A: “You are a hypocrite.”
B: “If you can’t name a specific time I have been hypocritical (and quote me), I am not hypocritical.”
Comments on Question
Can you please cite where and when Carl Sagan famously wrote or said, "lack of evidence is proof of nothing"? I cannot find a single instance where that is a direct quote. While he did write a great deal about the nature of evidence, and or, lack thereof, it had a very specific contextual application e.g. extra-terrestrial life, implying we could not conclude life on other planets is non-existent because we do not yet have the capability of determining that yet. He did indeed write that "absence of evidence is not evidence of absence", although, the quote is not exclusively his, nor does it mean the same thing as the misquote you are paraphrasing.
Answers
2Logical form:
P1: If one cannot name a specific time that X occurred, it did not occur.
P2: You cannot name a specific time that X occurred.
C: X did not occur (where X = "my hypocrisy").
This is the Argument from Ignorance. Just because something has not been proven, does not mean it is false - this only applies when we go looking for evidence and do not find it where we should. However, there is no suggestion that we've been through Person B's record to verify this. The argument is fallacious.
However, if Person A cannot substantiate their charge, they have made an unsupported claim.
I agree with the Argument from Ignorance . But I would like to stress a bit more that person A has the burden of proof here. Person B asking for an example is a perfectly reasonable request, but their conclusion is just too strong. If I were person B, my response would be:
“If you can’t name a specific time I have been hypocritical, then SHUT UP!” (said in a classic Adam Sandler tone)
or
B: “If you can’t name a specific time I have been hypocritical, then don't make the accusation. You are extremely accusatory - you do this all the time!"
A: "When have I accused you of anything besides this?"
B: "I can't think of any times off the top of my head."
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A's statement is a conclusion. No analysis needed. B's better response is to ask: based upon what evidence do you draw your conclusion. However, B said if you cannot provide evidence, I am not a hypocrite. The "I am not a hypocrite" is an error. As Sagan famously said, lack of evidence is proof of nothing. The best B can say is A does not have evidence to establish the proposition --- which may be true even though A lacks evidence.