 Hume’s omission of an equivalent rule has several implications for his philosophy and its relationship to Newton’s. First Hume replaces the authority of natural philosophy by his own criterion, the “copy principle”, when it comes to existence claims (see  section 2, of the entry on Hume).  Only the ideas that can be traced to a distinct impression will be permissible. Thus, Hume’s “science of man”, or “true philosophy”, offers to evaluate the claims of natural philosophy from a privileged position. This is the sense in which Hume is not a scientific naturalist.