You know of the disease called sleeping sickness. There also exists a sleeping sickness of the soul. Its most dangerous aspect is that one is unaware of its coming. That is why you have to be careful. As soon as you notice the slightest sign of indifference, the moment you become aware of the loss of a certain seriousness, of longing, of enthusiasm and zest, take it as a warning: your soul suffers if you live superficially.