Abtract: This article argues that the debate between reductive and nonreductive physicalists is best characterized as a disagreement about which properties are natural. Among other things, natural properties are those that characterize the world completely. All physicalists accept the “completeness of physics,” but this claim contains a subtle ambiguity, which results in two conceptions of natural properties. Reductive physicalists should assert, while nonreductive physicalists should deny, that a single set of low-level physical properties is natural in both of these senses. This way of drawing the distinction succeeds where previous approaches have failed and illuminates why the debate about reductionism is important.