Mainstream economic theory is not very clear about the forces that determine growth processes. It also has a positive aversion towards reckoning with some of the unintended consequences of production processes. This paper argues that since it was first proposed as part of welfare economics, the concept of "externalities" has been very inadequately dealt with, creating an inbuilt tendency within economic theory to neglect the environment in its theorisation of growth. Externalities it is argued, need to be restored to a central place, as factors very much internal to the economy. To interpret them within a Rawlsian framework of justice is to achieve a more complete theoretical comprehension of the ethics of technological change and its environmental impact.