A core idea behind EBM+ is evidential pluralism. This was formulated in the context of medicine by Russo and Williamson (2007) and further discussed by several authors (both to support – see for instance here – it and to criticise it – see for instance here). Clarke et al (2014) use the following formulation:

In order to establish that A is a cause of B in medicine one normally needs to establish two things. First, that A and B are suitably correlated—typically, that A and B are probabilistically dependent, conditional on B’s other known causes. Second, that there is some underlying mechanism linking A and B that can account for the difference that A makes to B.

Evidential pluralism of such type has also come to be known as RWT – the ‘Russo-Williamson Thesis.

An important clarification concerns the status of the thesis. RWT does not say what causality is, but how we come to establish causal relations. Philosophers call the first an ontological question and the second an epistemological question.

RWT engages with questions about evidence, evidence evaluation, and use, which have an important epistemological component. This means asking ‘what to look for’ in order to establish causal relations. RWT says that we look for dependencies and for mechanisms. These pieces of evidence are used to make inferences about causal relations, notably about their very existence, their plausibility, their strength, and about how they could be exploited for policy.

From this epistemological thesis about evidential pluralism, ontological pluralism does not follow straightforwardly. It does not follow from RWT that, if we need evidence of correlation and of mechanisms to establish causal relations, causation is itself constituted by correlations and mechanisms. The ontological question is currently unsettled. Russo and Williamson suggested here and here that the epistemic theory of causality offers a possible causal metaphysics. But there are other options worth investigating, such as informational accounts of causal production, as also explored by Illari here and Illari and Russo here.