This article argues that there is a fundamental contradiction between a profit-oriented economic system and long-term environmental sustainability. The `solutions' that are proposed by mainstream environmental economists as well as their `ecological economy' colleagues do not solve the central problems, but serve to further highlight the difficulties of changing capitalism towards sustainability. In a profit-oriented economy, capital accumulation is a prime driving force, and non-growth for the economy at large tends to result in serious economic and social crises. On the other hand, a de-coupling of economic growth from resource depletion and environmental degradation is possible only within certain sectors or product types and within relatively short time perspectives. The assumptions of mainstream economists about infinite economic growth (and infinite dematerialisation) represent a false ontology according to which the powers and mechanisms of the natural world are considered totally controllable by humans as if they were mere epiphenomena of the human world. On the other hand, the assumptions of certain ecological economists about the possibility of steady-state capitalism disregard the relation between capital and surplus value, which constitutes a strong mechanism driving the capitalist economy toward limitless growth.