The Russian novelist who wrote extensively about the psychological depths of humanity is Fyodor Dostoevsky. His works, such as "Crime and Punishment," and "The Brothers Karamazov," explore complex themes of morality, free will, and the human condition. Dostoevsky's characters often struggle with existential crises and moral dilemmas, providing deep psychological insight. His narratives delve into the darkness of the human soul while examining the interplay between faith and doubt. Through rich character development and philosophical musings, Dostoevsky has left an indelible mark on literature, influencing countless writers and thinkers.