I like this because it shows a very deep appreciation for the role of theory in science. Science isn't about exegesis of past scientists. And science isn't about being "right" in a deep, universal sense. It's about useful approximation and continual refinement. Keynes has been refined, but I'd probably say his tradition is still more useful than Barro would say it is. Still, the general perspective of Barro here holds up well.
2. "Man wants liberty to become the man he wants to become. He does so precisely because he does not know what man he will want to become in time." - James Buchanan, 1978 (HT - Don Boudreaux)
3. "Funeral by funeral, theory advances" - Paul Samuelson
Upon reflection, I think this one by Samuelson may be a necessary, but not sufficient condition.
4. "Economics is a technical and difficult subject. It is even becoming a science." - John Maynard Keynes, 1930