“The world is my idea.”
Schopenhauer opens his system by arguing that the experienced world is always presented in relation to a knowing subject, making perception and consciousness conditions of the world as it is known.
Source details
The World as Will and Idea, Vol. I, First Book, §1, Haldane and Kemp translation, Project Gutenberg #38427. “Idea” translates Vorstellung in this historical philosophical vocabulary and should not be read as the everyday sense of a passing thought.