Based on some earlier postings I think it is time to launch the concept of "reality interactivity index".
The index is a measurement of how interactive a certain environment is. An environment can have an index that is extremely low, which means that it does not react or change as a result of human actions. En environment can have an index that is extremely high, which means that every human action leads to a reaction and change in the environment. Of course, most environments are somewhere in between. One of the grand challenges for interaction design is to figure out what interactivity index fits a certain environment.
I am convinced that the reality interactivity index will become a common way of describing and defining the character of environments where humans live and act. It is a measurement that is similar to the notions of an environment being "natural" or "artificial", or "rural" versus "urban". We use these notions as a way to define the overall character of an environment. Interactivity will be, or maybe is already, one of the most important aspects of how we understand an environment.