Free will.

It's hard to accept, but free will is just not compatible with reality. It's like geocentrism. It seems obvious on its face because of our limited perspective, but nothing else in the universe makes sense if it's true. We live in a mechanistic universe and cause and effect doesn't suddenly stop when the atoms are part of a human.

I freaked out for about a week once I came to realize how much of our society is based on a scientific impossibility. Redesigning justice, ethics, healthcare, the very concept of blame, etc. to account for this is a daunting fucking task.