When we are alone we are not always busy; the labour of excogitation is too violent to last long; the ardour of inquiry will sometimes give way to idleness or satiety. He who has nothing external that can divert him must find pleasure in his own thoughts, and must conceive himself what he is not; for who is pleased with what he is?
 Samuel Johnson, The History of Rasselas, Prince of Abissinia (1759). copy citation

Context

“it is not pronounced madness but when it becomes ungovernable, and apparently influences speech or action. “To indulge the power of fiction and send imagination out upon the wing is often the sport of those who delight too much in silent speculation. When we are alone we are not always busy; the labour of excogitation is too violent to last long; the ardour of inquiry will sometimes give way to idleness or satiety. He who has nothing external that can divert him must find pleasure in his own thoughts, and must conceive himself what he is not; for who is pleased with what he is? He then expatiates in boundless futurity, and culls from all imaginable conditions that which for the present moment he should most desire, amuses his desires with impossible enjoyments, and confers upon his pride unattainable dominion.” source