But that's not a world. And again I get the feeling that you may mean something different by 'private'. To conjecture, means to hypthesize. But a hypothesis which can never be tested one way or another is not an hypothesis. In order for evidence to be evidence, there must be method for evaluating it.
Billiard balls are physical things.
Quarks are theoretical entitites. Quarks are based on a web of prior conceptual givens. To further clarify my ordinary attitude. I can't recall a time when I doubted that I, the world and others existed, though I did come to learn that Santa Claus was a wonderful fiction. Simply put, I cannot locate an originary point of learning.