 We believe that Hume’s discussion of the communication of motion by contact or impulse shows his debt to Newton especially clearly. In section 7, part 1 of the Enquiry Hume is criticizing the inherited ideas of necessary connection. We believe that both here and in section 4, part 1, where he rejects any a priori demonstration of causality, Hume is centrally concerned with the conception of necessary connection articulated by the mechanical natural philosophy. This philosophy had taken the communication of motion by contact or impulse as the paradigm of an a priori rationally intelligible causal connection, to which all other instances of causal connection must be reduced. The reduction would take place by reducing all observable causal relationships to the motions and impacts of the tiny microscopic parts of  bodies.[16]