Yep, a function of his job was to eliminate assumption and supposition from his testimony (Heinlein liked the idea enough to use it in a couple of later stories as well).

The primary difference between today's use of "expert" testimony and the extrapolated "fair witness" concept would be that the fair witness was strictly to be used as direct observation, and the expert witness today (seems to me) is used mostly to validate arguments based on points assumed to be out of the jury's knowlege base.

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