http://money.cnn.com/magazines/business2/business2_archive/2007/07/01/100117066/index.htm said:
From Michael the karate teacher, Allen learned the power of an empty mind when confronting new challenges. By running a travel agency, he picked up the idea for a "tickler file," a series of folders labeled by day, week, and month that helps people prioritize tasks. Many of GTD's most important concepts came from a small-business consultant named Dean Acheson, who taught Allen to create a master list of everything that commands his attention and assign a "next action step" for each.