Okay, so, in practice, we do know lots of things about the inputs. It may be impossible "in the general case," but engineering is done in the specific case. Any

Okay, so, in practice, we do know lots of things about the inputs. It may be impossible "in the general case," but engineering is done in the specific case. Anybody writing control software for spacecraft knows whether the mass of the moon or the spacecraft is larger, and can rearrange their equations accordingly.