Interaction is the relationship that exists between everything. Every object, in some way, is interacting with every other object to some degree at every point in time. In this way, interaction is not a switch that is either on or off. Instead, when looking at how any one object interacts with any number of other objects, one should evaluate it as a degree of interactivity. This understanding of interactivity is a derivative of the fundamental laws of physics. This understanding of interaction applies to more than just objects; it applies to systems as well – physical, digital, and cerebral. Of course systems are just complex relationships of multiple objects. At the root of all three of these systems is a series of physical objects that are interacting to drive the system. And at the root of all of these objects is a complex system that comprises the object. In this way I think that objects and systems are interchangeable.
Every object or system has a unique way of interacting with every other object or system. These interactions are commonly understood as physical phenomena, as some type of language, or a combination of the two. Examples of these interactions include a ball bouncing off of the pavement; two people communicating with words, hand gestures, and facial expressions; and a person using a computer to do work.