  The theoretical biologist Robert Rosen developed a highly original approach
for investigating the question "What is life?", the most fundamental problem of
biology. Considering that Rosen made extensive use of mathematics it might seem
surprising that his ideas have only rarely been implemented in mathematical
models. On the one hand, Rosen propagates relational models that neglect
underlying structural details of the components and focus on relationships
between the elements of a biological system, according to the motto "throw away
the physics, keep the organisation". Rosen's strong rejection of mechanistic
models that he implicitly associates with a strong form of reductionism might
have deterred mathematical modellers from adopting his ideas for their own
work. On the other hand Rosen's presentation of his modelling framework, (M,R)
systems, is highly abstract which makes it hard to appreciate how this approach
could be applied to concrete biological problems. In this article, both the
mathematics as well as those aspects of Rosen's work are analysed that relate
to his philosophical ideas. It is shown that Rosen's relational models are a
particular type of mechanistic model with specific underlying assumptions
rather than a different kind of model that excludes mechanistic models. The
strengths and weaknesses of relational models are investigated by comparison
with current network biology literature. Finally, it is argued that Rosen's
definition of life, "organisms are closed to efficient causation", should be
considered as a hypothesis to be tested and ideas how this postulate could be
implemented in mathematical models are presented.
