Full context:   In ancient Greece, metaphysics emerged in the 6th century BCE with the pre-Socratic philosophers, who gave rational explanations of the cosmos as a whole by examining the first principles from which everything arises. Building on their work, Plato (427–347 BCE) formulated his theory of forms, which states that eternal forms or ideas possess the highest kind of reality while the material world is only an imperfect reflection of them. Aristotle (384–322 BCE) accepted Plato's idea that there are universal forms but held that they cannot exist on their own but depend on matter. He also proposed a system of categories and developed a comprehensive framework of the natural world through his theory of the four causes. Starting in the 4th century BCE, Hellenistic philosophy explored the rational order underlying the cosmos and the laws governing it. Neoplatonism emerged towards the end of the ancient period in the 3rd century CE and introduced the idea of "the One" as the transcendent and ineffable source of all creation.[r]