Aristotle believed that every species has a role and purpose in the universe. Fulfilling this role well defines the ultimate good for that species. For humans, Aristotle identified characteristics that differentiate humans from other animals. He believed that when humans act according to their nature and fulfill their purpose, they feel fulfilled and happy. Happiness, according to Aristotle, is the greatest good because it is pursued for its own sake, rather than to achieve something else, and it indicates that one is living virtuously in accordance with their human nature.