To add to our excuses for not knowing everything, one might also say that the edge of our knowledge extends only when Y''''''ouActuallyAreGonnaNeedIt (YAAGNI). 

Need is perhaps the strongest catalyst for innovation and exploration of possibilities. What you already know is what you have thus far supposed that you will need to know. 

This changes the approach to that of "Y''''''ouCantLearnSomethingUntilYouAlreadyAlmostNeedIt".
* I'm not sure I would say that it extends "only" when you need it. I enjoy learning about certain subjects and do so just out of pleasure. I also have a relative who retains everything he reads, whether he needs it or not, whether he's even interested or not. Now, maybe there's a difference between information retained (photographic memory) and information that is ''known'' at an operational level. -- BrucePennington
* See: ''MaslowsHierarchyOfNeeds''
** Illustrated: http://0.tqn.com/d/psychology/1/G/h/5/hierarchy-of-needs.png
** About: http://psychology.about.com/od/theoriesofpersonality/a/hierarchyneeds.htm
* KnownBecauseItIsRemembered
* KnownBecauseItIsUsed