Mark Twain is a racist and was probably being explained onto why slavery was going to be abolished.
Anywho, the more you learn the less you know. When you learn about something like a field of study, you know of it but you lack all the new information that you are given to explore. To relate it to magic, you may be a master of card magic and are unaware of other forms of magic hence your knowledge is vast, that is until you learn about coin magic. That opens a door to more information that you don't know about which then makes your knowledge of both branches less then it once was.
Just my thoughts good luck trying to cypher that you get one hint.
Hint:
It's basically like the missions in COD, when you complete several quests and you think your mission is over, then the game throws at you some new information and new quests are added.