What is an essential question?

I’ve been thinking about Matt’s suggestion…
Then what IS an essential question? Should teachers guide students’ thinking? Does framing the reading limit students’ exploration of a topic?
I’m not sure, but Grant Wiggins talk about essential questions on his website, focusing on three components:

  • One meaning of “essential” involves important questions that recur throughout one’s life.
  • A second connotation for “essential” refers to key inquiries within a discipline.
    Essential questions in this sense are those that point to the big ideas
    of a subject and to the frontiers of technical knowledge.
  • There is a
    third important connotation for the term “essential” that refers to
    what is needed for learning core content. In this sense, a question can
    be considered essential when it helps students make sense
    of important but complicated ideas, knowledge, and know-how – findings
    that may be understood by experts, but not yet grasped or seen as
    valuable by the learner.

To what extent will students explore ideas on their own, and will they miss "essential" learning if not guided in some specific way? What happens when a student hears about Sir Thomas More in class and wants to know more but instead is led down a different path by the constraints of the class. Will he pursue this later or has the moment been lost?