That was my immediate thought regarding puzzle piece 4. To say "X could happen" is just to say "I am uncertain about X, or I don't have sufficient information to rule X out". The impossible (e.g., for me) is what I can rule out; the possible is what I can't. People talking about what's really, really possible and impossible are just talking about what they believe the closest-to-perfect (but still imperfect) agent could know.