… can read and understand several books with contradictory or complementary content without having their heads explode.
… and thus fear the one-book-religion as much as it deserves being feared.
… can understand, appreciate, and follow more than one methodology at the same time.
… know that no single book or methodology or language or tool or great tip will ever improve quality or output by any more than a couple of percentage points.
… know that any single book or methodology or language or tool or great tip used to the exclusion of common sense will reduce quality and output by close to 100%.
… truly understand the saying “A foolish consistency is the hobgoblin of little minds“.
… know the Orders of Ignorance and apply that knowledge.
… love writing lists of what real developers do and know.