The central concern of this paper is to re-evaluate Rosen's replicating (M,R)-systems, presented in his book 'Life Itself ', where M and R signify metabolism and repair, respectively. We look anew at Rosen's model of an organism in the light of extensive research into natural hierarchical systems, and the paper presents conclusions drawn from a comparison between Rosen's relational model and that of a birational complementary natural hierarchy. We accept that Rosen's relational model provides a useful stepping stone to understanding the nature of life, but also suggest that it induces potentially digressive conclusions. We conclude that a binary segregation of relational assemblies into mechanisms and organisms is insufficient, and indicate how a threefold segregation throws new light on Rosen's model. An organism is not 'the complement of a mechanism': the complement of a mechanism is its ecosystem. An organism is the 'complex interface' between mechanism and ecosystem.