In his work, Horgan argues for the compatibilism of agency, mental state-causation, and physical causal-closure. We generally assume a causally closed physical world that seems to exclude agency in the sense of mental state-causation in addition to physical causation. However, Horgan argues for an account of agency that satisfies the experience of our own as acting persons and that is compatible with physical causal-closure. Mental properties are causal properties but not identical with physical properties because there are different ontological levels. In this commentary, I shall reconsider the essential issues of this compatibilism (1), focus on a problem for Horgan’s conception of agent causation that arises from the causal argument for ontological reductionism (2), and propose to embed Horgan’s conception of agency within a reductionist approach in order to vindicate the indispensable character of agency (3).