‘Ideas’ explains the key ideas driving environmental politics. It begins with The Limits to Growth (1972) that questioned the long-term future of the Earth as a life-support system for humans. The concepts of ecological modernization, moral extensionism, ethics and the environment, deep ecology, and anthropocentrism are considered. It goes on to explain the ideology of ecologism and how it can be distinguished from conservatism, liberalism, socialism, feminism, and environmentalism. A central belief of ecologism is that aggregate growth must be reduced, and that this is very unlikely to be achieved by efficiency gains alone. The other core belief turns on the question of why (if at all) we should value the non-human natural world.