"Fun," says game designer Raph Koster, is just another word for "learning."

The idea that play is the best way to learn is not, admittedly, an entirely original idea. Even Plato, Koster is quick to point out, famously declared that "the most effective kind of education is that a child should play amongst lovely things." Still, few authors have explored the relationship between learning and play like Koster did in his 2004 book A Theory of Fun for Game Design.

The original edition of the book became something of a bible for game designers. University game design programs across the globe made it a part of their curriculum, and the book was translated into Japanese, Chinese and Korean, eventually selling over 30,000 copies.

This year Koster teamed up with publisher O'Reilly to release a 10th anniversary edition, due out December 5. The book's many charming illustrations are now rendered in full color, and Koster has updated the content to make it more relevant to the modern games industry, but the core idea at the center of A Theory of Fun – that learning and fun can be synonymous – has gone unchanged. That's mostly because in the 10 years since the book's release, nobody has been able to successfully challenge that idea.

Raph Koster, author of A Theory of Fun for Game Design

Much of Koster's game industry experience is with MMOs. He was lead designer on Ultima Online: The Second Age, and creative director of Star Wars Galaxies.

Six months ago he left an executive position with Disney to strike out on his own. Finding that he had enough time and money to do whatever he wants for a while, Koster began the Theory of Fun revision project. Koster has plenty of other creative projects on his plate, though; he says he's currently working on six original games on his own, including a card game about "making rainbows."

In other words, Koster has found a way to put the fun back into his own life, a challenge that faces every adult, and one which is addressed in A Theory of Fun.

After we grow up, Koster says, we sometimes wonder: "Where did the fun go?" Koster's argument is that adults have fully internalized many of life's most basic patterns – walking, talking, seeing and interpreting visual data. "After mastering a pattern," Koster says, "just using it isn’t necessarily fun. It’s pushing at the edges of it that's fun."

Kids have built up much smaller libraries of mastered patterns, so there's more stuff for them to go out and master. These are opportunities to have fun. "When you get older you have to actually go looking for stuff that gives you that sensation," says Koster.

The new edition of A Theory of Fun adds content that addresses games which are for purposes other than fun – games for meditation, or comfort, for instance.

One of the new color illustrations from the revised edition of the book. Image courtesy Raph Koster

Many other small details have been changed between the original and newer editions of the book. Research about cognitive differences between men and women, for example, has advanced significantly in the last 10 years. When discussing psychology, Koster also removed all references to the Myers-Briggs system, which has gone out of favor amongst psychologists, who now generally prefer the OCEAN model.

Another, funnier change: In the original edition of the book, there was a throwaway line about how nobody plays farming games anymore. "That," Koster says, has now "turned into a page-long riff about farming games and about how modern farming games teach business rather than farming."

Koster acknowledges that in a world as turbulent as the videogames industry, some parts of Theory of Fun may seem like obvious dogma, "a monolithic thing you rebel against." But the book still matters, Koster says, in part because nobody else has come along to knock it off its pedestal.

"Somebody really should," he says. "It's been 10 years, dammit."