Habits of thought (or unthought): I've encountered people who have difficulty grasping the concept of a necessary but not sufficient condition. Cheaters (say, in a relationship) sometimes believe being honest about (confessing to) their "mistake" is sufficient to re-establish intimacy or obtain forgiveness. Can't grasp the concept that honesty is a necessary but not necessarily sufficient condition for those goals. A lawyer I talked to said many of her clients have had trouble with grasping the distinction. Guess it comes from a self-centered view of negotiation--"I gave you this, so you must give me that." You can hear Logic staggering on down the street with his good buddies Abstraction and Objectivity (examining a problem from different viewpoints)--the same people will split hairs to explain why what they felt good doing was indeed good and what you do that makes them feel bad is indeed bad.