The main idea [of the ideal observer theory] is that ethical terms should be defined after the pattern of the following example: "x is better than y" means "If anyone were, in respect of x and y, fully informed and vividly imaginative, impartial, in a calm frame of mind and otherwise normal, he would prefer x to y.
Adam Smith and David Hume espoused versions of the ideal observer theory.
Notice that, by defining an Ideal Observer as omniscient with respect to nonmoral facts, Firth avoids circular logic that would arise from defining an ideal observer as omniscient in both nonmoral and moral facts.
A complete knowledge of morality is not born of itself but is an emergent property of Firth's minimal requirements.
For instance, to make a moral judgment about a case of theft or murder on Earth it is not necessary to know about geological events in another solar system.