A double crux is a technique CFAR refined and formalized. It is similar to a debate, but with a major difference: The goal is to simply pin down where exactly two people disagree. This can take a while. Even the best, most respectful debates are adversarial: it’s my opinion vs. yours, and we see which is stronger in an (ideally fair) contest. A double crux is collaborative: we’re just trying to find which is the exact point of contention here, so that if we go on to have an actual debate we won’t be talking past each other.