[1] http://en.wikipedia.org/wiki/Expert_system [2] https://probmods.org/ [3] https://probmods.org/conditioning.html#example-causal-infere...
But Norvig also put them in context, and left me with the impression that Expert Systems were proven inferior to Bayesian approaches for a lot of the problems they had been developed for.
I can well imagine, though, that there are problems where expert systems are a good fit.
As an interaction model and as a way of guiding control flow, as a weird and different way of handling user interaction and building extensible applications, I think the EMYCIN example from PAIP is pretty cool. I've wondered how possible it could be to use something like EMYCIN to write webapps ...