Quote by Terry Pratchett
My personal theory is that he has a very firm grasp upon reality, it's simply not a reality the rest of us have ever met before.
Summary
This quote suggests that the person being referred to is seen as having a solid understanding of the world, but their perception of reality differs from that of the majority. It implies that this individual's worldview or experiences are unique, perhaps unconventional or outlandish in comparison to what is generally accepted. It portrays them as someone who sees things differently, challenging the conventional notions of reality that most people are familiar with.