He was very willing to share his insights. He and I met on the old system dynamics mailing list. Through private emails, we discovered that we shared the belief that the engineering approach calls for varying levels of models, ranging from rules of thumb through the simple, highly-aggregated qualitative models all the way, in the appropriate cases, to carefully done simulation models. It's not the case that every problem demands detailed simulations, nor is it the case that a simple causal loop diagram (or influence diagram, as he preferred) always suffices. It was the same when I was an analog circuit designer: sometimes I would design a circuit based on simple rules of thumb, while other circuits called out for detailed analysis, and that had been his experience in his world, as well.
I learned a lot from Geoff: the rigorous use of influence diagrams, the use of QPID, the Analytic Hierarchy Process for decision making, and improved modeling skills at all levels of rigor. I read his Management Systems Dynamics, and I read and taught from his System Dynamics Modelling: A Practical Approach.
In recent years, we wrote less, for he was more focused on his latest book on mining, but I'm glad to have known him. I invite anyone who is interested in system dynamics to check out Tom's list of Geoff's writings. I certainly learned from them; perhaps you will, too.
Thank you, Geoff, for sharing your insights and engaging in dialog.