This is hardly an idle or a trivial question. Even now, neuroscientists, surgeons, philosophers, and psychologists continue to debate just what should be included as part of this blanket term. How much weight should be given each to components such as memory, creativity, motor skills, linguistic talent, reasoning, emotional capacity, homeostatic maintenance, and any other powers which are in some way regulated by the brain? Is there some fundamental aspect which somehow sets humans apart from other animals, or something intrinsic which divides animals from other forms of life?
The problem, essentially, is two-fold. First, the proverbial jury is still out debating the above considerations. Many feel that logic and language should be the primary, if not the exclusive, measures of intelligence. Others maintain that feeling and intuition should be considered further than they traditionally are. And all of these are just examples of how detailed the problems become. Second, when dealing with intelligence, we not only have to confront the differences in belief regarding humans themselves, but are also faced with considering the mental abilities of non-human species, a topic which some people think is not even worthy of further reflection. Only humans, such folks will say, truly have intelligence.
One critical aspect of intelligence which has received much more research and consideration in recent years is simple play, since it single-handedly entails creativity, motor ability, and often also utilizes memory, reason, and emotion. Some of the encouraging studies in the arena of play include dolphins who surf, dogs who begin play by bowing and then fetching, cats chasing various items, bison ice skating, hawks catching things in mid-air, chimpanzees using toys, ravens teasing other animals, parrots mimicking language, stingrays looping while swimming, whales singing and breaching, gorillas using sign language, elephants playing tug-of-war. And, of course, there are wolves using toys and chasing each other as well as other animals, without violence. Play uses cognitive and motor functions, and implies a release from boredom. Boredom, in turn, requires thought, the ability to plan and anticipate diversionary activities.
Of course, skeptics will continue to maintain that play is unworthy of scientific analysis, although it is presently one of the leading methods of assessing abilities across species. So perhaps we can also consider a more traditional aspect of intelligence: reason. Aristotle began a tradition over 23 centuries ago of describing humans as the "rational animals," and we seem loathe to relinquish this special ranking, even though it turns out to be arbitrary. Each species in the preceding paragraph has also shown the ability to not only make considered choices, but to weigh evidence before finalizing decisions, which requires not just thought but rational thought.
Playing and reasoning remain just two examples of what humans have used to describe themselves as somehow special or apart from other animals. Indeed, we have also emphasized opposed fifth digits, humor, "noble feelings," language, contemplation, religiosity, tool creation and usage, compassion, war, and the ability to experience orgasm as somehow unique to us, and every single thing on this list is displayed by multiple species.
Perhaps it is time instead, then, to consider how we are part of the natural world, rather than apart from it.