 Such an argument may not always be available. In fact, in the general case extrapolation from a model organism is intrinsically problematic. The main problem is what Daniel Steel (2008) has termed the “extrapolator’s circle”. The problem is this. We want to infer that some system S has a mechanism M because another system T is already known to have M. For this inference to be good, we must assume that S and T are relevantly similar. We can’t know that the systems are relevantly similar unless we already know that they both have M. But if we knew that, there would not be a need for extrapolating in the first place.