Problem-solving and particularly the part that focuses on root cause analysis (RCA) has always been one of the topics that has had my special interest. Specifically, two questions always slumbered in my head, viz. (1) whether you could speak of one root cause, or that you should speak of multiple (root) causes; and (2) whether you should speak of the root cause or rather the root condition. In a series of six posts, I will try to explain how rigorous problem-solving logic (using an example) can help us answer these questions. At the same time, I hope the example and the logic will be of use in your problem-solving efforts or your coaching thereof. This second post will introduce concepts like necessary condition, defensive and control barrier, and will apply these to our example.