Whether cells interact to form a organism or organisms to form an ecosystem, or gas molecules interact to form a weather front, the result is what people intuitively consider complex. People think they know it when they see it. Everybody talks about it. But nobody knows what it really is. In the absence of a good definition, complexity is pretty much in the eye of the beholder.
Human brains like to describe themselves as the most complex objects in the known universe.
We human beings like to think that we're very special. We bend over backwards saying we are the product of billions of years of evolution. But without good definitions, we're kind of shooting the breeze. Coming up with objective ways to measure this slippery quality would help extend science's reach into areas where prediction has proved difficult, if not impossible. That would mean understanding not only natural phenomena like weather but also the increasingly intricate machines people build.