Yeah, I was thinking about that after I wrote my comment. I suppose that rational could just be defined as &quot;what a person does&quot;, since they must have been trying to optimize for _something_.<p>But there must be some universal optimial for a given utility function, right?  Like: choosing to buy a BMW when your utility function is a looking for a cheap car is just simply irrational or a mistake.<p>Here&#x27;s a good one: All of my high school teachers (and my parents) encouraged me to study biochemistry in college. They assured me there&#x27;d be a good career for me afterwards.  But they were kinda wrong, and I had executed my utility function (what&#x27;s a major that will give me interesting and good work) on bad input.  I acted rationally, but would you say that my decision was a rational one?