" Don’t mention it,’ the sorcerer patted the neck of his horse, which had been scared by all the yelling from Yarpen and his dwarves. ‘To me, Witcher, calling killing a vocation is loathsome, low and nonsensical. Our world is in equilibrium. The annihilation, the killing, of any creatures that inhabit this world upsets that equilibrium. And a lack of equilibrium brings closer extinction; extinction and the end of the world as we know it.’ ‘A druidic theory,’ Geralt pronounced. ‘I know it. An old hierophant expounded it to me once, back in Rivia. Two days after our conversation he was torn apart by wererats. It was impossible to prove any upset in equilibrium. "
― Andrzej Sapkowski , Sword of Destiny (The Witcher, #0.7)