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."
