A Dubious Thesis:
Early liberal capitalist thinkers maintained the belief that acting in a self-interested fashion is to act rationally, and if everyone were to act rationally, then that is all that is needed for society as a whole to flourish.
A Really, Really Dubious Thesis:
Contemporary liberalism takes things a step further: Who’s to say what rationality even consists of and what “interests” are indeed “rational” to pursue? For society to truly flourish, people should just pursue whatever makes them feel good (so long as it’s all “harmless” and “consensual,” of course!).
And then people wonder why the world is a mess.