Robert Nozick, the recently deceased Harvard philosopher who wrote Anarchy, State, and Utopia which caused a huge stir around 25 or 30 years ago, presented the problem of being unknowingly hooked up to an "experience machine" which would flawlessly reproduce for you all the sensory input of real life. If I recall, the issue wasn't whether one could tell the difference between the machine and reality (it was assumed that one could not) but whether -- for the individual involved -- it really was different enough that he should care one way or the other.