A "neutral observer" is someone who is unbiased and doesn't take sides or have any personal interest in a situation. They look at things objectively and without favoring any particular side. Full definition
(ENTIRE BOOK) Four English theologians describe how Christianity appears personally to them, not as neutral observers, but as committed believers. (religion-online.org)
(This is true of most of the «sciences» because many «scientists» are not neutral observers — they have axes to grind.) (alephblog.com)