{ 1 } - someone who observes but never participates.
{ 2 } - an imaginary person of ideal moral wisdom -- a person who is fully informed and has impartial concern for everyone.
{ 3 } - an ethics teacher.

We'll never be ideal observers, since we'll always have some ignorance and bias. But the notion of an ideal observer is useful. It gives a vivid picture of moral wisdom and a way to understand the meaning and methodology of moral judgments.