The Natural Step (TNS) is a sustainability theory internationally promoted as a scientific framework for environmental management. Although the educational value of TNS is acknowledged, TNS theory includes implicit reasoning and value judgement as well as science. TNS implies zero growth in the physical scale of economies and companies, without stating this explicitly. TNS is rhetorical in its use of risk assessment, emphasizing a rise-in-concentration criterion for the sake of consensual argument, in the knowledge that it is inadequate for environmental protection. TNS has profound and valuable implications for contemporary economies, but users should be aware that TNS is primarily designed as a persuasive argument, and is not wholly science based.