Sussman anomaly

The Sussman anomaly is a problem in artificial intelligence, first described by Gerald Sussman, that illustrates a weakness of noninterleaved planning algorithms, which were prominent in the early 1970s.

While the significance/value of the problem is now a historical one, it is still useful for explaining why planning is non-trivial.

In the problem, three blocks (labeled A, B, and C) rest on a table.

The problem starts with B on the table, C atop A, and A on the table: However, noninterleaved planners typically separate the goal (stack A atop B atop C) into subgoals, such as: Suppose the planner starts by pursuing Goal 1.

Sussman (and his supervisor, Marvin Minsky) believed that intelligence requires a list of exceptions or tricks, and developed a modular planning system for "debugging" plans.