Affordance is the perceivable possibilities for action seen in objects. Perception theorist J. J. Gibson claims that we perceive possibilities for action. i.e. surfaces for walking, handles for pulling, space for navigation, tools for manipulating, etc. In general, our whole evolution has been geared toward perceiving useful possibilities for action.
Diagnosing the ramification of this theory in the functionality of objects and furthermore spaces, provides us with the understanding that human requirements for inhabiting spaces are beyond the systematic means of Corbusiean measurement, which in essence has provided the base for accepted building code standards around the world.
This theory also illuminates the field of human-space interaction and the interpretation of architecture as interface, which will be discussed in another post.