Dag Kittlaus, the co-founder and CEO of the company that created Siri, has decided to leave Apple, four years after Apple acquired Siri for $200 million.

Siri is the voice-activated virtual personal assistant found on the iPhone 4S, which helps users write and send texts and e-mails, set alarms and reminders, schedule meetings, place calls, surf the Web, dictate notes, play songs and even answer complicated, context-sensitive questions.

Kittlaus' departure was amicable and planned for some time; he reportedly wants to take time off to spend with his family in Chicago and pursue new entrepreneurial ideas, the company said.

Originally, Siri was a free downloadable App Store application for the iPhone. The third-party app was born from SRI's CALO project, or Cognitive Assistant that Learns and Organizes, which was the largest artificial intelligence project in U.S. history. DARPA invested $150 million into CALO over five years.

Kittlaus, Siri's CEO since 2007, continued to spearhead Siri's development even after his company was acquired by Apple. Before that, he was an entrepreneur-in-residence at Stanford Research Institute and enjoyed a stint at Motorola.

Even as an app, Siri had more than 30 different partners with built-in APIs, including OpenTable, MovieTickets.com, StubHub, Rotten Tomatoes, Bing, Google Maps and Yelp. Siri could help its user find a taxi, search Google, and even book a reservation, which is something it can't do now. The Siri app even resembled what users see on the iPhone 4S, from the microphone button on the bottom of the screen to the menu that displays a list of viable functions.

When Apple bought Siri, most of the changes were cosmetic--the green and white theme was replaced with a sleek silver and purple design--but Cupertino, Calif.-based Apple also substantially improved the voice recognition software, tethered in applications and added the Wolfram Alpha knowledge engine.

The Siri team will move on without its Norway-born co-founder, although other key members from Siri, including co-founders Adam Cheyer and Tom Gruber, will reportedly stay at Apple.