This article revisits the payments for environmental services (PES) concept and reviews existing PES definitions. Based on Weberian philosophy of science, it is argued that an ideal PES type, strongly embedded in PES theory, is needed to understand their logic. Many broader, empiricist definitions fail to distinguish PES from the larger generic family of positive environmental incentives, thus eroding their meaning by excessive vagueness. Arguably, PES definitions should focus on describing a functional tool, rather than normatively integrating desirable PES outcomes. A modified narrow PES definition is proposed, outlining conditionality as the single defining feature, avoiding the buyer-seller terms, and linking PES to offsite externalities. Extensive explanatory guidelines address many valid conceptual concerns raised in the recent PES literature.