You might think of static members as "class" members, and non-static members as "instance" members.
A class can be loaded simply by a call to one of its static members, but that (in itself) does not create an instance. Within a static context, there is no guarantee that any instances exist, so non-static (instance) members cannot be referenced.
"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer sscce.org