"In economics, the crucial Keynesian concept is uncertainty. Where it prevails, the simple rules of classical economics don’t apply. That’s because the classical economics that both predated Keynes and superseded him relies on rational actors making rational assessments. In order to make such assessments you have to have reliable knowledge, usually derived from past experience. Buyers of oranges or newspapers or legal services can be said to possess such knowledge. Buyers of speculative securities cannot. They’re always looking into an uncertain future, “anticipating what average opinion expects the average opinion to be,” as Keynes put it. This, in Skidelsky’s convincing telling, is why financial markets are so prone to disorder and disaster. It’s not that investors are terribly irra­tional. It’s that no one can really know what rational means when it comes to pricing investment securities" - Thomas Brox Røst from Bookmarklet