Economist: "And if you look here, I'm going to draw a line on this piece of paper. This line is intuitively correct."

Student: "So it's reflected in the real world data?"

Economist: "Well no..."

Student: "Oh, is it the result of an experiment performed in ideal conditions?"

Economist: "Er, those actually say something else..."

Student: "Oh. So where does it come from?"

Economist: "Some guy 50 years ago said it had good vibes, so we just go with this."