The paper concerns the possibility of naturalistic ethics. It seems that ethics properly understood must be described as a discipline necessarily focused on evaluation of human action, analyzed in the light of its moral value. Such evaluation, in turn, presupposes an agent who is responsible for his or her actions. So understood agent seems impossible without agent's being capable of self-determination in action. According to the naturalistic thesis, however, such freedom is impossible; all human action is causally determined, with no place for 'sui generis' causation, and such a thesis must be interpreted as excluding the possibility of responsibility for one's action. If so, then the concept of naturalistic ethics seems contradictory. Nonetheless, some authors (ex. Frankfurt and Dennett) try to show that the concept of moral responsibility (so crucial to ethics) does not necessarily entail freedom understood as 'sui generis' causation, and therefore it is compatible with determinism. In this paper I analyze their argumentation with the purpose to assess its conclusiveness. The conclusion I reach is that responsibility postulated by Frankfurt or Dennett is to be understood as merely epiphenomenal, as such it must be treated more like an illusion than a real property of human beings. Therefore, the thesis that naturalistic ethics is a contradictory concept seems to maintain its soundness.