 Sometimes it is easy to see what explains a supervenience thesis. Consider the examples of trivial supervenience relations from  Section 3.5.  It is obvious why there cannot be an A-difference without a B-difference if A is a set of necessary or impossible properties—namely, because there cannot be an A-difference at all. The supervenience relation is explained by the necessity or impossibility of the supervening properties. It is also obvious why A-properties supervene on A-properties. Of course, property identity claims are sometimes not a priori knowable; consider, for example, the fact that the property being water is identical with the property being H20. Still the fact that being water is identical with being H20 explains why there cannot be a difference with respect to being water without a difference with respect to being H20. And supervenience with only nomological necessity can be explained by appeal to laws of nature. It is in virtue of the Wiedemann-Franz Law that electrical conductivity supervenes with nomological necessity on thermal conductivity.