ABSTRACT
The title of this chapter involves the terms “interpretation of probability” and “causal model,” which will not be familiar to many readers. However, the meaning of these expressions will be explained as we go along. To give these explanations, we must begin by making some general observations about the notion of causality. Causality is a key concept in medicine, but, to analyze this concept, it will be useful to begin by distinguishing between theoretical and clinical medicine. What could be called theoretical medicine consists of a body of laws and theories, many of them involving causality, which have been discovered and then confi rmed by medical research. A typical accepted causal law is the following:
The varicella zoster virus (VZV) causes chickenpox (1)
This kind of causal claim is described as generic , because it covers many cases. In clinical medicine , however, a doctor examines a particular patient and has to find out what
causes that patient’s symptoms. A doctor may, for example, decide that the rash of Miss Anne Smith, aged 4, is chickenpox and so caused by VZV. This is an instance of single-case , rather than generic, causality. Some authors speak of type/token causality instead of generic/singlecase causality, where type = generic, and token = single-case.
