  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 must insist that the constant conjunction between the two of them is not of the right sort for the relation between them to be one of causation. Hume thinks that there is causation between two events just in case there is a law linking them and also thinks that there is a law linking two events just in case they are constantly conjoined. Reid's first objection shows that it is not the case that wherever there is no constant conjunction there is no law. His second objection shows that it is not the case that wherever there is constant conjunction there is a law. Constant conjunction is neither necessary nor sufficient for the presence of a genuine law. The hard problem that remains for the Humean, then, is to produce criteria for distinguishing genuine laws from regularities that are not laws at all.