A "Fair Witness" is a fictional profession in the book "Stranger in a Strange Land" by Robert A. Heinlein. In the book, Fair Witnesses were individuals trained to see the world around them, as literally as possible, until sensed (see/hear/smell/taste/touch) otherwise.

Another way to describe it would be someone that takes all things sensed as literal as possible & have no reason to believe what they just sensed will remain that way when they are no longer sensing them (seeing/tasting/etc.)
An excerpt from the book itself, where the character referenced as "Anne" is an off-duty Fair Witness:

Jubal to Jill: "Even Cavendish did not--at least he won't say so. You know how Fair Witnesses behave."

Jill: "Well...no, I don't. I've never met one."

Jubal to Jill: "So? ANNE!"

Anne was on the springboard; she turned her head. Jubal called out, "That house on the hilltop--can you see what color they've painted it?"

Anne looked, then answered, "It's white on this side."

Jubal went on to Jill: "You see? It doesn't occur to Anne to infer that the other side is white, too. All the King's horses couldn't force her to commit herself...unless she went there and looked--and even then she wouldn't assume that it stayed white after she left."
by ZoLatKam March 11, 2014
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Fair Witness is a fictional profession invented by Robert Heinlein. (Stranger in a Strange Land)

A Fair Witness is an individual trained to observe events and report exactly what he or she sees and hears, making no extrapolations or assumptions. An eidetic memory is a prerequisite for the job, although this may be attainable with suitable training.

In Heinlein's society, a Fair Witness is a highly reputable source of information. By custom, a Fair Witness acting professionally, generally wearing distinctive white robes, is never addressed directly, and is never acknowledged by anyone present.

While Fair Witnesses may hold jobs, at all times they must honor the traditions of the Fair Witness and do their best to avoid any possibility of bias when wearing the robes and repeating the truth of what they have seen, done or heard.

Fair Witnesses are prohibited from drawing conclusions about what they observe. As a demonstration, in the novel a Fair Witness was asked to describe the color of a house in the distance. She responds, "It's white on this side"; whereupon Heinlein explains that she would not assume knowledge of the color of the other sides of the house without being able to see them. Furthermore, after observing another side of the house she would not then assume that any previously seen side was still the same color as last reported, even if only minutes before.
You can trust what he says. He's the closest thing to a "fair witness" of anybody I've ever known.
by Deacon John Hubertz July 15, 2017
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Fair Witness-originating from the book 'Stranger in a Strange Land' by Heinlein, Fair Witness was a made up profession, and their job was to record everything they heard/saw. What it means play/be fair witness is to, well take questions people ask you somewhat literally, as seen in the example. Yes, you can do something, but that doesn't mean you will-and that can lead into you will/would do something but not right now because no time was specified. This pretty much means messing with peoples' words/questions until they ask you in a very specific way.
James: Okay, can you pick up all of your crap?
Michael: Yes
James: WELL?
Michael: Oh, I'm sorry-did you want something?
James: Stop playing fair witness a pick this stuff up!
by AlliterationAnne September 4, 2009
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