Philosophy has always progressed by being aware of its past, and it has been said that the legacy of the ancient Greek thinkers to Western philosophy is nothing less than Western philosophy itself. The ancients invented our subject, and Plato and Aristotle are still widely regarded as the two greatest philosophers of all time. A full philosophical education demands some understanding and critical engagement with key aspects of their wide and powerful thought. These great thinkers and their predecessors, the earliest Greek philosophers, raise fundamental questions and develop the use of argument to try and answer them: What is the nature of reality? Is the world the way we perceive it to be? Is the world constantly changing or always the same? What is it to be a good person? What is the best kind of life for a human being to lead? What it is to know what is good? What is it to know anything, rather than just have an opinion?