Person you're replying to is using an "absence of evidence" fallacy as their argument, also known as an "appeal to ignorance" [0]. They're inferring that the ab

Person you're replying to is using an "absence of evidence" fallacy as their argument, also known as an "appeal to ignorance" [0]. They're inferring that the absence of evidence that Obama didn't use his BlackBerry "for Secret, Top Secret, or other classified communications" is potentially evidence that he did in fact do so. (I would have replied to him directly, but the comments have since been [appropriately] flagged) In reality, no argument could ever be made if you had to prove the negative of every argument. Some other common applications of this fallacy off the top of my head: "Well we don't have proof that children weren't trafficked in Comet Pizza, so it's proof that it did actually happen." "We don't have proof that no kids used litterboxes at school, so it's proof that they did use litterboxes." [0] https://en.m.wikipedia.org/wiki/Argument_from_ignorance https://en.m.wikipedia.org/wiki/Argument_from_ignorance