The Humean account of causation faces the hard problem of distinguishing genuine laws from mere regularities that are not laws.

The problem is made clearer by Reid's second objection to Hume's analysis of causation and constant conjunction. He writes, “It follows from [Hume's] definition of a cause, that night is the cause of day, and day the cause of night. For no two things have more constantly followed each other since the beginning of the world” (EAP 4.9, 249). Since we don't ordinarily think that day is the cause of night, or vice versa, Hume must deny that the two are actually constantly conjoined, or, rather, he m