He was a blogger who used to blog on controversial issues with somewhat heterodox opinions (not since 2017 anymore though) and generally blogged about interesting stuff, related to cognition and psychiatry; I was somewhat of a fan and thought he was a great writer. He attracted a large community, some of who were a bit, well, let's say "controversial", but it is also one of the most interesting communities I have ever encountered. He was part of the rationalist ideology/movement.

He also wrote Unsong, which is a very interesting work of fiction and blogged about various other stuff.