Hannibal Lecter: First principles, Clarice. Simplicity. Read Marcus Aurelius. Of each particular thing ask: what is it in itself? What is its nature? What does he do, this man you seek?
Clarice Starling: He kills women...
Hannibal Lecter: No. That is incidental. What is the first and principal thing he does? What needs does he serve by killing?
Clarice Starling: Anger, um, social acceptance, and, huh, sexual frustrations, sir...
Hannibal Lecter: No! He covets. That is his nature. And how do we begin to covet, Clarice? Do we seek out things to covet? Make an effort to answer now.
Clarice Starling: No. We just...
Hannibal Lecter: No. We begin by coveting what we see every day. Don’t you feel eyes moving over your body, Clarice? And don’t your eyes seek out the things you want?
-Silence of the Lambs
I don’t follow what this has to do with rationality. Could you explain further?