For over twenty years now I've been mulling over the following problem: how do you broadly define/measure intelligence? Probably someone has already said that (or come to the same conclusion) but here's how I see it. Intelligence can be measured as the level of abstractions that a certain entity can operate with. The most primitive life forms, e.g. bacteria just have raw inputs and reactions. Zero abstractions. As you move up along the intelligence ladder you'll find that more intelligent organisms can operate with more and more abstract (laden) concepts/ideas. This is true of humans as well.
Now this conclusion/hypothesis doesn't answer the question whether something is intelligent or not but it looks to me that anything that can respond to stimuli and change its reaction to the same stimuli over time can be considered intelligent.
This is not a question/problem per se, I'm looking forward to your opinions about this hypothesis.