To a poet nothing can be useless. Whatever is beautiful and whatever is dreadful must be familiar to his imagination
 Samuel Johnson, The History of Rasselas, Prince of Abissinia (1759). copy citation

Context

“I ranged mountains and deserts for images and resemblances, and pictured upon my mind every tree of the forest and flower of the valley. I observed with equal care the crags of the rock and the pinnacles of the palace. Sometimes I wandered along the mazes of the rivulet, and sometimes watched the changes of the summer clouds. To a poet nothing can be useless. Whatever is beautiful and whatever is dreadful must be familiar to his imagination; he must be conversant with all that is awfully vast or elegantly little. The plants of the garden, the animals of the wood, the minerals of the earth, and meteors of the sky, must all concur to store his mind with inexhaustible variety;” source