The central question of the theory of industrial ecology (IE) is whether IE is/will be a form of descriptive or prescriptive science. Some debate the same (more or less) question by asking whether IE is objective or normative. This author argues that in the case of the physical flows of matter and energy the description of the ecosystem flows can produce an important prescription for the industrial ecosystem overall goal and vision of ecological sustainability. However, the ecosystem cannot tell us what to do in practice and we cannot prescribe concrete solutions or practical measures for industrial ecosystems based on ecosystem description. In the case of the structural and organisational characteristics and properties of industrial ecosystems, it is very difficult (or impossible) to derive anything prescriptive for industrial ecosystems from ecosystem description. Responses to this contribution are encouraged for publication in future issues of Progress in Industrial Ecology: An International Journal (PIE).