In 1994, when I first proposed the do-calculus, I selected these three rules because they were sufficient in any case that I knew of. I had no idea whether, like Ariadne’s thread, they would always lead me out of the maze, or I would someday encounter a maze of such fiendish complexity that I could not escape. Of course, I hoped for the best. I conjectured that whenever a causal effect is estimable from data, a sequence of steps using these three rules would eliminate the do-operator. But I could not prove it.
