 The precautionary principle is often believed to be a general instruction to be cautious. However, this is not what is usually meant by the term “the precautionary principle”. This is a principle that is defined in international treaties and in European legislation. According to these definitions, the precautionary principle concerns how we should act when it is scientifically reasonable to suspect a risk to health or the environment, but the evidence is not strong enough to show conclusively that the risk exists. The precautionary principle says that in such cases we may, and often should, take measures against the potential danger. This definition has been the standard at least since 1992, when the precautionary principle was included both in the United Nations’ Rio Declaration and in the European Union’s Treaty of Rome, which is now called the Treaty on the Functioning of the European Union (Pyhälä et al., 2010). Its practical application depends on the interpretation of scientific evidence that indicates the presence of a danger, but is not strong enough for scientists to treat the existence of that danger as known. The evaluation of such evidence raises methodological and epistemological issues that are in need of further clarification, not least with conceptual tools developed in the philosophy of science.