This paper argues that the policy principle of ecologically sustainable development - first and famously articulated in the Brundtland Report of 1987, Our Common Future - is an impossible principle. As a guiding principle, it demands that we must simultaneously maximize three different things: social justice, ecological sustainability and economic development. However, this is impossible to do. Despite the principle - and the closely associated idea of 'triple bottom line accounting' - being nonsensical, it has been maintained because it serves a number of other ends. It provides psychological comfort, it helps to maintain the status quo of business-as-usual neoliberal capitalism and it provides status-rewarding employment for the professional class. For true ecocentric sustainability on 'spaceship' Earth, we need to reject 'sustainable development' and build an ethically founded eco-socialism.