Date: September 28, 2001
From: Sampsa Hautaniemi, NIH

Dr. Pearl,
I am at the halfway of your book Causality, which I think to be excellent and instructive. What I am writing to you is that I browsed to homepage of Dr. Lotfi Zadeh and found out that he has a Word document whose subject is 'CAUSALITY IS UNDEFINABLE'. The page is I know you have answered many times reports like that, so if there is already discussion going on of this subject I would appreciate the URL/name of the journal as I am really interested in the subject.

Author's Reply:
Dear Sampsa,
Thanks for writing to me. I am puzzled by Zadeh's claim that 'CAUSALITY IS UNDEFINABLE'. For me, the adequacy of a definition lies not in abstract argumentation but in whether the definition leads to useful ways of solving concrete problems. The definitions of causal concepts that I have used in my book have led to useful ways of doing things, and to meaningful assessments of the risks associated with doing things one way or another (e.g., choosing treatments, or adjusting for confounders). If Zadeh were to present us with alternative definitions or with alternative ways of solving causality-related problems, we would be able to judge their merits. But until this happens, claims of "undefinability" leave me mildly puzzled, somewhat amused, but scientifically unmoved -- there are urgent problems out there awaiting solution.

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