The phenomenon that is rarely discussed in the art world: a new job on the gallery wall does not necessarily have to be painted by an artist who is signed. Some well-known artists such as Damien Hirst and Jeff Koons, openly recruit a small army of assistants to do his paintings and sculptures. Other rental assistance quieter.

The art market insiders say soaring prices and demand for contemporary art encourages the use of interns and more artists.The art world is divided into practice: While some collectors and dealers put a premium on paintings and sculptures the artist made his own hand, others say that assistants are a necessity in today's market.

"The artist has to make a choice," said Mark Moore, owner of Mark Moore Gallery in Santa Monica, Calif. "They either hire an assistant, or risk not being able to meet its obligations to its dealers. And art market, which is fickle and sensitive gets the impression that the artist has disappeared from the world of art.

What makes a masterpiece? What is it about a piece that makes the hair stand on end, the blood boil, the pulse race and the eyes widen in wonder? Why are some paintings so effective and so unapologetically and brutally authentic that walking into a room with them, you can feel their presence humming off the walls, even before your eyes have crossed the room to try and take them in?

This is the question Thames and Hudson are trying to answer in a new book that features contributions from dozens of artists, historians and critics on what constitutes a truly great piece of art. Seventy submissions are richly illustrated with in-depth essays in this 300-page glossy behemoth of a book. Philip Pullman is getting all excited about Edouard Manet; Quentin Blake is driven to distraction by Degas; Christopher Dell (who edited the book) is bewitched by Rembrandt; and Martin Kemp (the learned professor, that is, not the acting Spandau Ballet-er) plays to the crowd and plumps for Leonardo da Vinci, in an essay that Christopher Dell reckons is a “groundbreaking” interpretation of the most famous picture in the world.