One does not discover new lands without consenting to lose sight of the shore for a very long time.
--- Andre Gide. Quoted widely, but I have not found a source citation. Some versions have "new oceans" or "new continents" in place of "new lands". It's not in The Oxford Dictionary of Quotations, or Bartelby.com's collection of quotation references. It may be apocryphal.
Update 21 Oct 2007: Bret Battey, with a little help from books.google, found the reference: The Counterfeiters. In 1951 Knopf version in books.google.com, either Page 49 or 326 (can't tell for sure from the listing).