only so much as is demanded

The co-operation of the beholder, ...as demanded for the enjoyment of a work of art, depends partly upon the fact that every work of art can only produce its effect through the medium of the fancy; therefore it must excite this, and can never allow it to be left out of the play and remain inactive. This is a condition of the aesthetic effect, and therefore a fundamental law of all fine arts. But it follows from this that, through the work of art, everything must not be directly given to the senses, but rather only so much as is demanded to lead the fancy on to the right path; something, and indeed the ultimate thing, must always be left over for the fancy to do. Even the author must always leave something over for the reader to think; for Voltaire has very rightly said, "Le secret d'être ennuyeux, c'est de tout dire."

-- Arthur Schopenhauer The World as Will and Idea
Translated by R.B. Haldane and J. Kemp