Padgham & Winikoff (2005) agree that an intelligent agent is situated in an environment and responds (in a timely, though not necessarily real-time) manner to environment changes. However, intelligent agents must also proactively pursue goals in a flexible and robust way. Optional desiderata include that the agent be rational, and that the agent be capable of belief-desire-intention analysis. Some 20th-century definitions characterize an agent as a program that aids a user or that acts on behalf of a user. The influential "" (2009) defines an agent as "anything that can be viewed as perceiving its environment through sensors and acting upon that environment through actuators", and characterizes intelligence as the ability to succeed at acting in accordance with certain ideal standards for rationality.