Jaegwon Kim's exclusion argument was originally developed as a critique of anomalous monism and non-reductive physicalism. However, its influence has extended far beyond this context and is now frequently interpreted as a general argument against higher-level causation under physicalism. This paper argues that such a generalization depends upon a particular conception of higher-level organization. Within Kim's framework, higher-level organization is exhausted by supervenient entities alone, and no additional physical constituents are admitted at the higher level. We argue that this conception does not obviously exhaust all logically possible forms of higher-level organization. In particular, one may consider hybrid constitutions that combine higher-level entities with additional lower-level physical constituents. The aim of this paper is not to defend the existence of such constitutions, but to examine what justifies their exclusion. Once the possibility of hybrid constitutions is recognized, the exclusion problem is transformed. Rather than asking why higher-level causation should be excluded, we must first ask why additional physical constituents should be excluded from higher-level organization. We suggest that this exclusion does not follow straightforwardly from physicalism itself. Instead, it appears to require an additional assumption that treats higher-level and lower-level entities as belonging to distinct ontological categories. In this sense, the exclusion argument may rely upon a hidden dualism of levels. ( direct link )