Nietzsche in "Human, all too human"

Belief in inspiration. Artists have an interest in others’ believing in sudden ideas, so-called inspirations; as if the idea of a work of art, of poetry, the fundamental thought of a philosophy shines down like a merciful light from heaven. In truth, the good artist’s or thinker’s imagination is continually producing things good, mediocre, and bad, but his power of judgment, highly sharpened and practiced, rejects, selects, joins together; thus we now see from Beethoven’s notebooks that he gradually assembled the most glorious melodies and, to a degree, selected them out of disparate beginnings. The artist who separates less rigorously, liking to rely on his imitative memory, can in some circumstances become a great improviser; but artistic improvisation stands low in relation to artistic thoughts earnestly and laboriously chosen. All great men were great workers, untiring not only in invention but also in rejecting, sifting, reforming, arranging.

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