Concurrently, though separately, I've been catching up with Jurgen Habermas, a German philosopher perhaps best known for his concept of communicative rationality. The idea, as far as I can tell, is that rational commentary is created by dialogue between people. In other words, to meet the standard of rationality, it's not sufficient that I observe something about the world and report it to other people. Rather, if I'm talking with someone else and we both fully understand something and agree with it, we've created a bit of rationality in the world.
Another way to explain this is to take Jane in "Coupling" as a counterexample. She's perfectly rational according to some conceptions. Her conclusions proceed logically from her premises, even inexorably. And her premises are always sincere, and usually comprehensible in themselves (as when she explains that she eats meat, despite being a vegetarian, because being vegetarian doesn't mean you don't eat meat). However, she seldom achieves consensus over the validity of her claims, as seen in the short collection of clips below. From the Habermasian perspective, Jane is an irrational human being. The fact that the other characters consistently perceive her irrationality (despite being farcical themselves) suggests an underlying sympathy with Habermas's ideas.