The importance of downward causation lies in showing that it shows that functional properties such as mental properties are real, although they cannot be reduced to physical properties. Kim rejects nonreductive physicalism, which includes leading functionalism, by eliminating downward causation, and thereby returns to reductionism. In this paper, I make a distinction between two aspects of function—functional meaning and functional structure and argue that functional meaning cannot be reduced to the physical level whereas functional structure can. On this basis, I further distinguish between the integer of the function, which includes the functional meaning and the functional structure, and the whole of the functional realization, and also among the strong, medial, and weak supervenience relations. So-called downward causation is indeed the relationship between the whole of the functional realization and its physical realizer, which is a whole-part relation instead of a relation between levels. As a result of understanding downward causation in this way and abandoning the principle of the causal closure of the physical world, Kim’s argument becomes invalid and nonreductive functionalism, justified.