Thought Experiment Thursday: Cow in the Field

Anxious that he has lost Bessie, his cow, the farmer raises an eyebrow.
The milkman says don't fret after having seen the cow.
But the farmer must confirm that standing in his field is the black and white Bessie, eating grassy chow.
The farmer indeed sees a black and white cow shape and is satisfied.
The milkman checks and sees that Bessie is in fact there, but behind a tree, seeming to hide.
Curiously though, in front of the tree are sheets of black and white paper that are tied.
What did the farmer really see?
Tricks of the mind; can it be?
For surely, he only saw the paper in front of the tree.