 What does this structural equation mean? There are in fact competing interpretations. Pearl (2000) regards the structural equations as the conceptual primitives of his framework, describing them as representing ‘the basic mechanisms’ of the system under investigation. However, for the purposes of exposition, it is more convenient to follow the interpretation of Woodward (2003) and Hitchcock (2001), who think of the structural equations as expressing certain basic counterfactuals of the following form: