 Let’s leave aside, for the moment, the ultimate metaphysical status of “ultimate causes and principles” in Hume’s thought. He is willing to accept that some limited set of fundamental scientific laws (i.e., “ultimate causes and principles”) are available to human researchers. (Hume hedges his bets a bit; note his use of “probably”.) Moreover, these four principles are all associated with Newton’s project; the mechanical philosophy had only accepted communication of motion by impulse as an explanatory principle. Of course, Hume treats these laws not so much as describing genuine “causes” or “powers” of nature, but rather—in opposition to Newton’s own view which insisted on the reality and causal nature of forces (Janiak 2007)—as a way to keep track of the appearances (recall  section 3  above).