Isn't rationality a consensus between specific beings, and doesn't rationality change? What you and I find rational today may not be the case for someone else from a different time and/or environment (it's only in the consensus between you and me are the others irrational). If you agree with this, then you can't take rationality as an absolute.
Uhhh, no. Very much no. There are technical measures for the performance of decision-making processes. If you've got access to the same information as someone else, and they make more accurate predictions than you do, they're almost certainly doing better on the whole "use information to figure out how reality works" project.
The only sense in which "rationality" changes is when you're talking about the project to use our squishy meat-based thinky bits better, cultural and environmental baggage and all. In which case, "rational" strategies are going to be much different if you have, say, lots of shame-based baggage due to a strict Catholic upbringing.
Also, please stop listening to postmodernists until you've got a thorough nuts-and-bolts understanding of how systems work. It's really not good for you.
I'll give a more concise definition of what rationality is: a measure of how well beings use information to model, predict, and act upon reality. Having a consensus that you predicted reality well neither changes your prediction nor reality. The reported scores are subjectively determined - you can cheat and bribe the judges - but it doesn't change the underlying objective facts of the situation.
