 This research was supported by the European Research Council through Starting Investigator Grant No. 640638 (J.S.), the Italian Ministry for University and Research through PRIN project “From Models to Decisions” (J.S.) and a research fellowship of the Alexander Humboldt Foundation (N.W.). The authors would like to thank the editors for their invitation to contribute to the Stanford Encyclopedia of Philosophy, Reuben Stern for helpful feedback, and Judea Pearl for extensive comments on a previous draft. The authors have no conflicts of interests.