Iran must be building a bomb, therefore the negative must be proven for the assertion to be wrong. There is no evidence, ergo that proves the argument. The theory that Iran must have a bomb originates in Argument From Adverse Consequences, proceeds to an Argument By Uninformed Opinion ends up being Argument Ad Nauseam. While absence of evidence is not evidence of absence, after a decade of intense scrutiny by the world's intelligence agencies and the IAEA, all we have is an absence of evidence, Tautology, Assuming The Answer, aka Begging the Question, is rampant amongst those who believe what they wish and refuse to apply intellectual tools developed over millennium to scrutinize their arguments. The IAEA quite correctly states that it cannot give Iran a clean bill on the question of a "Possible Military Dimensions." The mere absence of evidence is not evidence of absence. Thus simply because no one has produced anything like proof of an Iranian bomb program we cannot on the data conclude there is none. Nor by any means is the absence of evidence proof that a bomb program exists.
North Korea leader Kim Jong Un hails accord with South as landmark (Reuters)
from the article: Dagan brought a possible attack on Iran closer