I'm sure there is a 'rational' [='sensible, reasonable'] explanation for his decision.
Humans are 'rational' beings. [=are able to use reason when thinking about something]
No 'rational' [='sane'] person would behave like that.
(..) But tomorrow I might be absolutely sure that it's five. It's precisely that possibility that makes tomorrow intriguing. "Reason only satisfies man's rational requirements. Desire, on the other hand, encompasses everything. Desire is life." Yeah, well, I don't know all about that... (..)1974 The Gambler