A Probabilistic Safety Analysis expresses uncertainty about the possible future damaging consequences of complex installations, such as chemical or nuclear plants, in terms of probabilities. Often these probabilities are interpreted as measures of physical properties of the installation and how it is operated, and a PSA is seen as a means to carry out these measurements. This view is useful for making comparative statements about the riskiness of installations, particularly when comparing them with standards. It is argued here, however, that this interpretation of probability is inconsistent with all the standard philosophical theories of probability, and so a different interpretation of PSA is necessary. We suggest, alternatively, that by using the standard subjective theory of probability, PSA may be seen as a tool for argument, rather than an objective representation of truth. In this interpretation the problems of expert choice and model validation become less problematic.
