Abstract
Russo and Williamson (2007) argue that, to properly establish causal claims in the health sciences, evidence of two kinds is required and neither is sufficient on its own. Firstly, it requires evidence that the putative cause makes a difference to the effect—which in medicine is most usually evidence showing a correlation between putative cause and effect. Second, it requires evidence about the existence and the nature of a mechanism by which the cause is supposed to produce the effect. In this project, I evaluate and reject this “Russo-Williamson Thesis” (RWT), as it is often called, then seek to salvage a more nuanced version of it. I argue that these types of evidence are disjunctively necessary (at least one is required for causal inference) and not conjunctively necessary (both are required, as in the original RWT). I argue that the root cause of the problem with the RWT is that it does not adequately distinguish between different possible conceptions of mechanism, which I seek to mend with a thorough examination of the historical and contemporary articulations of mechanistic thought. I go on to show that if one articulates the notion of mechanism more carefully, it is possible to articulate a defensible version of (something like) the RWT, as a disjunctive rather than conjunctive thesis. I further distinguish between different uses that the different kinds of reasoning have, by distinguishing between establishing and understanding causal claims in the health sciences. I propose that establishing is important for prediction and understanding is important for explanation. I then argue that evidence of mechanisms is not a prerequisite for prediction, since (because of the disjunctive thesis) it is not a prerequisite for establishing causal claims. However, understanding goes beyond establishing, and I argue that mechanism is central to understanding and explanation. Thus while the RWT in its original form is too crude, it has a kernel of truth, which I believe is captured by the disjunctive RWT plus the insight that mechanism is central to understanding.