Trygve Haavelmo (a Norwegian economist and Nobel laureate), who in 1943 advocated equation modification to represent interventions. Nevertheless, Spirtes’s translation of equation deletion into the world of causal diagrams unleashed an avalanche of new insights and new results. The back-door criterion was one of the first beneficiaries of the translation, while the do-calculus came second. The avalanche, however, is not yet over.
