The paper aims to analyze some aspects of the interaction between ethics and meta­physics as separate parts of philosophical knowledge. A good example of this kind of in­teraction can be found in case of moral criticism of D. Lewis’s modal realism. The study has two aims. On the one hand, the author aims to show how effective and convincing moral argument can be against some metaphysical theory. On the other hand, there are several arguments against the idea to rely on moral arguments when one chooses between competing metaphysical theories. These arguments are presented and analyzed. In con­clusion the author points at some problematic aspects of the relations between ethics and metaphysics.