Robert Rosen's central theorem states that organisms are fundamentally different from machines, mainly because they are “closed with respect to effcient causation.” The proof for this theorem rests on two crucial assumptions. The first is that for a certain class of systems (“mechanisms”) analytic modeling is the inverse of synthetic modeling. The second is that aspects of machines can be modeled using relational models and that these relational models are themselves refined by at least one analytic model. We show that both assumptions are unjustified. We conclude that these results cast serious doubts on the validity of Rosen's proof.