What are counterfactuals?

Have you ever wondered where you would be today if you had chosen something different in your life? Moved to another city 10 years ago? Studied art? Dated another person? Taken a motorcycle trip in Hawaii? Answering these types of questions requires us to create alternative worlds, worlds that we have never observed. If you’ve ever tried doing this for yourself, you already know intuitively what counterfactuals are.

Let’s try to structure this intuition. We can think about counterfactuals as a minimal modification to a system (Pearl, Glymour, and Jewell, 2016). In this sense, they are similar to interventions. Nonetheless, there is a fundamental difference between the two.

Counterfactuals can be thought of as hypothetical or simulated interventions that assume a particular state of the world (note that interventions do not require any assumptions about the state of the world). For instance, answering a counterfactual question such as &...