He's written an entire book [0] on how to add notions of causality to the existing algebraic framework for expressing correlation. He alludes to the core idea in the next part of the interview:

"Mathematics has not developed the asymmetric language required to capture our understanding that if x causes y that does not mean that y causes x. It sounds like a terrible thing to say against science, I know. If I were to say it to my mother, she’d slap me."

His home page [1] links to several presentations (e.g. [2]) where he lays out the key ideas. [0] "Causality: Models, Reasoning and Inference", Pearl

[1] http://bayes.cs.ucla.edu/jp_home.html

[2] http://bayes.cs.ucla.edu/IJCAI99/ijcai-99.pdf