“Musing takes place in a kind of meadowlands of the imagination, a part of the imagination that has not yet been plowed, developed, or put to any immediately practical use. [--] [T]ime spent there is not work time, yet without that time the mind becomes sterile, dull, domesticated. The fight for free space—for wilderness and for public space—must be accompanied by a fight for free time to spend wandering in that space.”