What is the main conflict in Foundation and Earth by Isaac Asimov?

Foundation and Earth is the story of one man's quest to justify a decision he's already made. The story begins with Golan Trevize asking why he had decided that all of humankind should become part of Gaia. Gaia is a planet of a single consciousness, a single super organism, in a sense. Trevize gradually explains that he has made that decision while discarding the idea of another plan, one that is never explained. That plan, devised by Hari Seldon, depends on two fundamental truths - that there be sufficient numbers of people involved so that statistical data can be projected, and that the people not be aware of the plan's projected outcomes.