Originally Posted by Myth
I'd say that Walt is smart, not rational. A rational person doesn't cook meth when they are diagnosed with cancer.
He was losing his mind in season 5. Remember his response to Mike when asked why he thinks the magnet thing has worked. Also, the whole "Say my name" scene and killing Mike because of his ego, realizing only later that it wasn't necessary. I think they've tried to portrait Walt as increasingly losing touch to reality in S5.