The Book of Why: The New Science of Cause and Effect

by Judea Pearl

He was too talented for any of his high school math teachers to keep him interested. What he eventually accomplished was truly amazing. Verma finally proved what became known as the d-separation property (i.e., the fact that you can use the rules of path blocking to determine which independencies should hold in the data). Astonishingly, he told me that he proved the d-separation property thinking it was a homework problem, not an unsolved conjecture! Sometimes it pays to be young and naive. You can still see his legacy in Rule 1 of the do-calculus and in any imprint that path blocking leaves on rung one of the Ladder of Causation. The power of Verma’s proof would have remained only partially appreciated without a complementary result to show that it cannot be improved. That is, no other independencies are implied by a causal diagram except those revealed through path blocking.