What do people's actions tell us about their intentions? Consider the simple situation of a person standing in front of a door, digging into his pockets, and then fumbling with a set of keys. We can easily assume by his actions that the person is looking for the right key to open the door to the room which he will then enter. Until the person actually finds the key and opens the door, we understand what the person is trying to do, or what his intentions are, as a matter of inference from somewhat uncertain information. After all, people can feasibly perform many actions when they stand in front of a door, fumbling with a set of keys. Yet our vast experience, both personal and vicarious, with standing-in-front-of-door activities provides us with the necessary back ground knowledge to make a reasonable inference about the person's likely intentions in the door-opening scene, even if this person wasn't necessarily trying to communicate anything by his or her actions.

This simple example demonstrates how our understanding of human behavior depends on our ability to make inferences about people's intentions. All actions can be described in purely physical terms, but we rarely talk of human actions in this way. For example, suppose I see Mary waving her hand. To describe Mary's action in purely physical terms, I might say something like Mary raised her arm slightly, extended her fingers, and moved her forearm and hand from side to side, etc.