  Does a dualist need to accept (CN), however? The notion of a causal nexus has come under criticism, often from philosophers working in the Humean tradition (Blackburn 1990). More generally, (CN) and kindred principles might be thought to rest on a conception of causality that is now obsolete, finding no place in modern physics (for further  discussion, see  the metaphysics of causation,  §2). But the next three versions of the problem can arise even for those who reject the need for a causal nexus.