An end-to-end calibrated process. Photography as a physical process is very quantifiable so with a imaging chain you can know that your outputs match the inputs within a known error margin. Typical modern systems are good to a delta-E of about 2, which is a not perceptible to humans. Claiming that a RAW file is abstract because you can't see it is silly; it's a collection of numbers that have formal meaning. There doesn't need to be a human present and directly absorbing the information for the information to be real.
Originally Posted by ic-racer
Do some research on monitor and printer calibration, it's a very well-understood (though rarely by photographers) domain. Suffice to say, we can make extremely accurate visual recordings of reality, and we can quantify exactly how accurate they are.