In this paper, my aim is to clarify the connection between Hume’s theory of causality and the PSR (the principle of sufficient reason) by analysing Leibniz’s view of the PSR and Hume’s arguments. I focus on Hume’s two arguments about causation from A Treatise of Human Nature and an argument and an endnote from An Enquiry concerning Human Understanding. I begin with the PSR, then proceed to a difference between cause and reason, and arrive at Hume’s arguments and scholarly discussion.