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."