A quote about the significance of Portraiture in the field of art...?

I'm a portrait artist, and someone I know ( I hesitate to call him a friend at this time) suggested portraiture was not as difficult or valuable as art that is created completely from the imagination. When I think of artists such as Rembrandt and Leonardo Da Vinci this notion seems quite ridiculous. I would like to reference a very respectable source to retort his statement. If someone knows of a particular quote or page I could send him I would be very happy. Thanks.