Knowledge

We used to believe that we were gaining in knowledge over time, but with the unification of information theory with thermodynamics, we now understand that all knowledge comes with a price: that for every new bit of information you obtain, you must spend a bit of what you already knew.
We understand that now; but what particular idea we paid for that understanding has been forgotten. Perhaps it was one of Grimm's fairy tales, or the tune to an old song, or the face of a childhood friend.