The relationship between cognitive psychologists and researchers in artificial intelligence carries substantial benefits for both. An ongoing investigation in causal reasoning in medical problem solving systems illustrates this interaction. This paper traces a dialectic of sorts in which three different types of causal resaoning for medical problem solving are identified in verbatim protocols and simulated by computer program. The first, and most commonly discussed, type of causal reasoning consists of "causal links" holding between states of the world. The second is based on "qualitative simulation" of systems of continuous parameters related by qualitative constraints. The third, the "one parameter simulation", is a hybrid of the first two. These three types of causal descriptions are not alternative hypotheses, but apparently coexist in the expert's mind. (BS)