It's interesting that that "quotation" from Albert Einstein, a sentiment I would love to attribute to him, because it encapsulates an idea I share about the importance of creativity, seems never to be traced to one of Einstein's actual writings. Has anyone ever checked the latest edition of Quotable Einstein
to see if that is a genuine Einstein quotation? In the English-speaking world, Einstein is second only to Mark Twain in having sayings attributed to him that he never said.
No single thing is a "true sign of intelligence", that is to ignore the mutual interdependence of things. You can't have imagination without any knowledge.
Jeff Hawkins has a good general framework for intelligence. X is more intelligent than Y if X can make better predictions given the same information as Y. X is more intelligent than Y if X can make predictions as good as Y's with less information.
to make sure you were referring to the author whose book I have perused before. I will note for the record that Hawkins has not won wide acceptance for his view of intelligence among other researchers to date.
That sounds like a good proposal on a definition of intelligence, but it doesn't tell you why someone would be good at predicting something, which is what my point was: that when you try to claim that any one thing is the reason for someone's ability to, in this case, predict something, you're ignoring the multitudes of other reasons that are all interrelated.
All the various forms of intelligence can be seen as a form of predictive capability. For example, coordination of many elements or one's own body parts involves making moves which avoid chaos, collision, or some other dysfunction. Avoidance is a kind of prediction.
— Albert Einstein http://quotationsbook.com/quote/21310/
(and Isaac Asimov had a lot of imagination)