It's worded a bit ambiguously, but I think it means we have to deal with the knowledge of how much we don't know about something and when making policy, every new think you don't know is a point of contention that can be argued over. In some cases that's beneficial, because it keeps us from making a mistake, in others it's detrimental, because it keeps us from making the beneficial change, but if all policy decisions start tending towards infinite argumentation as more and more things we don't know the answers to are linked to the topic, that's also a problem.