This phrase is a result of a discussion regarding the movie 300. At the end of the film, Leonidas throws a spear at Xerxes, and manages to graze his face with it, cutting, but not killing him. A debate sprang up about whether or not Leonidas meant to kill him and failed, or purposefully meant to only make him bleed. Patrick and the hosts were in the former camp, while some listeners fell into the latter group.

The main point that the listeners made regarded the comment Leonidas made about him showing that even a god-king, Xerxes, could bleed, and therefore mortal. Obviously a spear to the head would have made Xerxes bleed just as well, if not more, as the cut he sustained, but then he would not have been conscious of this fact.

Patrick made the very logical argument that killing Xerxes would prove without a doubt that he was mortal, and that it doesn't matter if he is conscious of that fact or not. He pointed out that it would be very prudent to kill the king of an oppressive enemy nation in war time, and that Leonidas would have meant to kill Xerxes. The fact that he managed to hit Xerxes at all is a testament to his skill and some luck, given his fatigue from battle and distance from Xerxes. Had Leonidas only been aiming to simply graze Xerxes, he could have "missed" and actually killed him, or missed him completely. While Patrick made his case, he became worked up at the lack of logic other people employed.

Even after Patrick had stated his position, and the majority of people had agreed with him, he would still receive emails and calls arguing otherwise. While some of them may have been simply to get the reaction from Patrick, he would again argue the point that Leonidas did mean to kill Xerxes, and would again become agitated, as with Skitchy's email in Episode 174: Promo School. The callers eventually reduced their argument down to simply stating that Leonidas meant to miss to prove that Xerxes was mortal. Then, to mock this argument, listeners began to call in about news and other stories, and would tie them into "proving it was mortal." Comments like these would frequently achieve laughter, given the ridiculous nature of the original argument as well as the scenario it was currently being applied to.

The debate about 300 first appeared on Episode 172: Learn Things, and notably, Patrick's rant on the illogical thinking of fans also inspired the resurfacing of the famous "learn things" credo. While the "trying to prove he was mortal" line consequently never reached catchphrase status of "learn things," though it's used every so often to lampoon Internet know-it-alls. Successful use of this line is demonstrated in the following posts from the NLO Forums: 1, 2, 3.