The phrase "impartial arbiter" means a fair and unbiased person who is responsible for making judgments or decisions in a conflict or dispute. Full definition
ALJs serve as impartial arbiters at regulatory and benefits - granting agencies. (resume-place.com)
You should also be careful to take into account the vested interests of constituencies that might try to persuade you to accept certain viewpoints, as they are less than impartial arbiters of truth. (moneysense.ca)
The court can not fulfill its democratic function as an independent and impartial arbiter between government and the individual, or between individuals, if the government limits those who may come before the court by means of financial or procedural deterrents. (slaw.ca)