"Critical systems thinkers like Midgley identify three waves of systems thinking over the last 50 years or so. Early systems theorists (e.g. Bertalanffy) described systems in physical terms, resorting to metaphors from electronic computation or biology. This 'hard systems' tradition still has its advocates and practitioners... Subsequently the limits of the physical metaphor... were reached, and the second wave of systems thinking developed. This 'soft systems thinking' employed social metaphors to develop appropriate systems approaches for human systems. The move to a more phenomenological, interpretative understanding of human systems, where meaning is central and is negotiated intersubjectively, parallels the new paradigm / crisis of social psychology of the 1970s. The Third wave, or critical systems school, in which Midgley locates himself, has drawn on the critical theory of Habermas, particularly in relation to theories of knowledge and of communicative rationality, and on the work of Foucault and followers on the nature of power."