I was in my 101 course when a spry young student piped up "Dr.OP, I don't think it is rational to assume all individuals operate rationally"
Wow. Stunning. I froze in front of the class, entirely blindsided by this novel critique on a field I had dedicated my life to. I stuttered before reaching a reasonable explanation on why the assumption of rationality was necessary and carrying on with my class.
I spent the entire day sitting in my office pondering her comments.
I realized she was right. How can we assume rationality in a world where people don't drink coffee?
Undergrad made comment that shook me to my core


So that's the 2018 Nobel then... her for the theorem and you for the proof by contradiction.
I was in my 101 course when a spry young student piped up "Dr.OP, I don't think it is rational to assume all individuals operate rationally"
Wow. Stunning. I froze in front of the class, entirely blindsided by this novel critique on a field I had dedicated my life to. I stuttered before reaching a reasonable explanation on why the assumption of rationality was necessary and carrying on with my class.
I spent the entire day sitting in my office pondering her comments.
I realized she was right. How can we assume rationality in a world where people don't drink coffee? 
I was in my 101 course when a spry young student piped up "Dr.OP, I don't think it is rational to assume all individuals operate rationally"
Wow. Stunning. I froze in front of the class, entirely blindsided by this novel critique on a field I had dedicated my life to. I stuttered before reaching a reasonable explanation on why the assumption of rationality was necessary and carrying on with my class.
I spent the entire day sitting in my office pondering her comments.
I realized she was right. How can we assume rationality in a world where people don't drink coffee?Theorem: Everyone drinks coffee.
Proof: A set of people E consists solely of irrational individuals K and irrational individuals K', K ∪ K' = E. Clearly, all rational individuals drink coffee, so the proof is trivial if K' = ∅. Assume the existence of individuals who do not like coffee, i.e. K' ≠ ∅. Then there exists an X ∈ K' which is irrational, and hates coffee. But because x is irrational, they act irrationally and drink coffee despite disliking it, hence they are rational. This is a contradiction and proves the theorem.
Corollary: Everyone is rational.