Experience has borne out that members of the first machine class are the ones which we should consider reasonable models of computation in the course of formulating the Cobham-Edmonds Thesis. It is also widely believed that members of the second machine class do not provide realistic representations of the complexity costs involved in concretely embodied computation (Chazelle and Monier (1983), Schorr (1983), Vitányi (1986)). Demonstrating this formally would, however, require proving separation results for complexity classes which are currently unresolved. Thus while it is widely believed that the second machine class properly extends the first, this is currently an open problem.