Interviewer: Inferring causal effects from data involves many steps. Where do you think your work fits within this overall process?Pearl: I seek to understand the conditions under which such inference is theoretically possible, allowing of course for partial scientific knowledge to guide the inference. My focus has been on a class of models called "nonparametric" which enjoy two unique features: (1) They capture faithfully the kind of scientific knowledge that is available to empirical researchers and (2) they require no commitment to numerical assumptions of any sort. Leveraging these models, I have focused on the problem of identification, rather than estimation. This calls for transforming the desired causal quantity into an equivalent probabilistic expression (called estimand) that can be estimated from data. Once an estimand is derived, the actual estimation step is no longer causal, and can be accomplished by standard statistical methods. This is indeed where machine learning excels, unlike the identification step in which machine learning and standard statistical methods are almost helpless. It is for this reason that I focus on identification -this is where the novelty of causal thinking lies, and where a new calculus had to be developed.
Historical PerspectiveInterviewer: What is your perspective of the history of the causal inference movement, and how the movement came to where it is today?Pearl: My perspective comes through the lens of a computer scientist. I look at this movement as a struggle to develop a mathematical language for capturing cause effect relationships, so that we can express our assumptions faithfully and transparently, derive their logical implications and combine them with data. It's really a wedding between two non-intersecting languages, one is the language of cause and effect, the other is the language of data, namely statistics (Pearl, 2019b).The wedding occurred quite late in the history of science because science had not been very kind to causality. It has revolved around the symmetric equality sign '=' of algebra, and thus deprived us of a language to capture the asymmetry of causal relationships. Such a language was developed in the past three decades, using graphs, and it now enables us to answer causal and counterfactual questions with algorithmic precision.Graphs are new mathematical objects, unfamiliar to most researchers in the statistical sciences, and were of course rejected as "non-scientific ad-hockery" by top leaders in the field (Rubin, 2009). My attempts to introduce causal diagrams to statistics (Pearl, 1995;Pearl et al., 2000) have taught me that inertial forces play at least as strong a role in science as they do in politics. That is the reason that non-causal mediation analysis is still practiced in certain circles of social science (Hayes, 2017), "ignorability" assumptions still dominate large islands of research (Imbens and Rubin, 2015), and graphs are still tabooed in the econometric literature (Angrist and Pischke, 2014). While most re-