Most mechanists recognize two main aspects of mechanistic explanation: etiological and constitutive. Salmon (1984) describes them as two different ways of situating an explanandum phenomenon in the causal nexus (see also Craver 2001b; Glennan 2009). Etiological explanations reveal the causal history of the explanandum phenomenon, as when one says a virus explains a disease. Constitutive explanations, in contrast, explain a phenomenon by describing the mechanism that underlies it, as when one says brain regions, muscles, and joints explain reaching.