In Defense of Unification (Comments on West and Koch’s review of Causality)

A new review of my book Causality (Pearl, 2009) has appeared in the Journal of Structural Equation Modeling (SEM), authored by Stephen West and Tobias Koch (W-K) West and Koch (2014). I find the main body of the review quite informative, and I thank the reviewers for taking the time to give SEM readers an accurate summary of each chapter, as well as a lucid description of the key ideas that tie the chapters together. However, when it comes to accepting the logical conclusions of the book, the reviewers seem reluctant, and tend to cling to traditions that lack the language, tools and unifying perspective to benefit from the chapters reviewed. The reluctance culminates in the following paragraph: “We value Pearl’s framework and his efforts to show that other frameworks can be translated into his approach. Nevertheless we believe that there is much to be gained by also considering the other major approaches to causal inference.” W-K seem to value my “efforts” toward unification, but not the unification itself, and we are not told whether they doubt the validity of the unification, or whether they doubt its merits. Or do they accept the merits and still see “much to be gained” by pre-unification traditions? If so, what is it that can be gained by those traditions and why can’t these gains be achieved within the unified framework presented in Causality? In their explanation of why they do not embrace the unification whole-heartedly, they seem to be speaking about another book entirely, not the one I have written. In their words: “[In Pearl’s framework] there are relatively few unknown causal influences in the system, the units tend to be homogeneous, causal effects are assumed to be stable across settings and time, and levels of treatments can be set to specified values as reflected in the do(x) operator...” This description would come as a total surprise to readers of Causality, because none of the listed shortcomings apply to the methods described in the book, as I will soon elaborate, and as would be attested by anyone who tries to solve concrete problems using several alternative methods. To the best of my knowledge, chapter 7 of Causality (especially pp. 231– 234) is still the only text available in which concrete problems are solved side by side in both