Philosophy is the art of rational thought.
Every branch of knowledge depends upon rational thinking, but philosophy is unique in that the application of reason and logic becomes the very source of knowledge - whether employed critically, in the examination of the assumptions underlying a field of inquiry or belief system, or constructively in the search for necessary truths about the nature of reality itself. Each individual who discovers philosophy repeats in his or her own life the story of the very beginnings of philosophy. The ability to theorise, to derive knowledge by a process of reasoning, struck the earliest thinkers in ancient Greece as an almost magical power. They constructed theories about everything in the universe. Then the doubts set in.