We think we have scientific knowledge when we know the cause. (Aristotle, Posterior Analytics Book II, Part 11) About 12 years ago, when I was a graduate student, many people were concerned about my Ph. D. topic - investigating the effect of the complexity of procedu- lized tasks on the performance of human operators working in nuclear power plants. Although they agreed with the fact that procedures (especially emergency operating procedures) play a crucial role in securing the safety of nuclear power plants, it was amazing that most of them pointed out a very similar issue: "I cannot understand why operating personnel see any difficulty (or complexity) in condu- ing procedures, because all that they have to do is to follow a simple IF-THEN- ELSE rule as written. " Actually, this issue is closely related to one of