Time.
Among all things, nothing seems so poignant a mark of intelligence as the time it takes for an intelligent agent to effect something compared to the time it takes for non-directed actors. Consider a hundred coins all dropped out of a bag onto the floor, repeat the drop enough times and eventually you will get one case where all one hundred coins are either heads or all tails. However, give the bag to a person and ask them to take the hundred coins and choose either heads or tails and lay all one hundred coins on the floor with the same side up and the time it takes that person will be far less than the time it takes using the random method.
This example seems obvious, but how do we define the obvious? How do we scientifically categorize that the difference is due to the acting will and intelligence of the person? Time seems to be the tool mark, certainly.
Beyond that, though, I'm stumped. Ideas?
