Mental Activity and Physical Exercise – Wilhelm von Humboldt

"The most spiritual men, as the strongest, find their happiness where others would find their destruction: in the labyrinth, in hardness against themselves and others, in experiments. Their joy is self-conquest: asceticism becomes in them nature, need, and instinct. Difficult tasks are a privilege to them; to play with burdens that crush others, a recreation. Knowledge-a form of asceticism. They are the most venerable kind of man: that does not preclude their being the most cheerful and the kindliest." Friedrich Nietzsche

"To succeed in any undertaking, any art or any trade or any profession, simply keep it ever persistently fixed in mind as an aim, and then study to make all effort toward it play or recreation. The moment it becomes hard work, we are not advancing." Prentice Mulford