Anything having to do with my teeth.
The idea of being strapped down/restrained.
Vivesection and the idea of anything being skinned or given surgery while still alive / without anesthetic.
Stories of people who wake up in the middle of surgery under anesthetic who cannot move or scream even as they feel everything.
The idea that everything, including morality, is arbitrary and that all we do is for naught, that all human value is dependent upon the whim of what other humans think of you/me and that existence is meaningless. (Though I am comforted by the thought that, even if this turns out to be true, that we are all equal in Death).
Sleep tight, kids.