“Mistakes are the portals of discovery.”
What Joyce wrote, in the National Library chapter of Ulysses, is: “A man of genius makes no mistakes. His errors are volitional and are the portals of discovery.” The words are Stephen Dedalus's, in the middle of his Shakespeare argument. The short form drops the speaker, the genius, and the claim that the errors are deliberate — which is most of the sentence's meaning.
- — James Joyce, Ulysses (Project Gutenberg #4300) (1922)

