This paper views the concept of natural capital from an economist's perspective. It begins by drawing on historical debates in economics on the nature of capital. These serve to identify central issues to do with the relationship between theory, empirics and method in the way in which the concept of capital is deployed in economic discourse. It is suggested that these have resonance for current discussions of the concept of natural capital. Against this background, the paper then discusses the way in which natural capital figures in the analysis of sustainability, and pinpoints various incoherencies. Finally, it draws on recent analyses of technical innovation as a possible solution to the problem of sustainability. It is suggested that there may be a conflict between narrow path-dependent solutions to the alleged problem, and more open learning-based approaches. The latter are exemplified by building on and reinterpreting the environmental economist's concept of a quasi-option.