[dcwca]

>If you know you're not under drug usage and if there are other witnesses of the event, then you'll have a very high degree of confidence into the idea that the apple indeed fell on the ground, so much confidence that you would consider it a fact.

As soon as you start talking about what happened with those other witnesses, the group begins influencing the way each other remember what happened, and the narrative becomes more "real" than the actual memory. The more time that passes, and the more times the story of the apple falling from the tree is told, the more reinforced the narrative becomes, regardless of how the apple got to the ground.