In his testimony before the Leveson Inquiry on Monday, former British prime minister Gordon Brown denied ever having a phone call in which he allegedly threatened Rupert Murdoch, essentially accusing Murdoch of lying under oath.

Brown didn't actually use the word "liar," but that's what he meant when he told the inquiry, "this conversation never took place." He went on, according to The Guardian's Dan Sabbagh and John Plunkett: "I'm shocked and surprised that it should be suggested, even when there's no evidence of such a conversation, that it should have happened...  I didn't phone – I didn't return calls to News International. I didn't phone Mr Murdoch, I didn't talk to his son, I didn't text him, I didn't email him." 

Brown was referring to a phone call Murdoch described in his April testimony, in which Murdoch said an "unbalanced" Brown promised to "declare war" on Murdoch's News Corporation after The Sun sided with Brown's political opponents the Conservatives on Sept. 30 2009. Murdoch stood by his testimony, News Corp. told reporters from The Guardian and CNN on Monday.

In April, Brown said Murdoch was "wholly wrong" about that phone call, so it shouldn't come as a surprise that he denied it in his testimony on Monday. But the significance here is that both men have made factually contradicting statements under oath, which means one of them is lying. They're not disagreeing on something abstract, such as the nature of their relationship, they're disagreeing about an event that either did or didn't happen. Now all they have to do is come up with some proof.