The paper deals with von Mises’ metaphysics and argues that his methodological dualism concerns only his epistemology. The framework of Mises´s ontology is materialistic monism. Although Mises strongly criticizes materialism, his critique does not concern metaphysical ontological materialism as long as it does not try to eliminate the specific method of the social sciences. In this sense Mises’ metaphysics is fully naturalistic – it does not include any “spiritual” agents and postulates a world consisting exclusively of elementary physical particles. The study illustrates this point on Mises’ determinism and his rejection of the free will as well as on his Darwinian account of the evolution of human mind. The specific method of the social sciences which makes use of the category of finality serves merely as an efficient heuristics and is based on a pragmatic view of science. It allows us to describe the irreducible complexity of social phenomena that cannot (at least not yet) be grasped by the physicalist vocabulary alone. Thus the argument of complexity is seen as a cornerstone of Misesian methodological dualism.