Platonism is an ancient philosophy which has had great impact on Christianity. It suggests that Matter and Idea are two extremes of "being," with Idea being the more pure and closer to the Form of the Good, or the Platonic equivalent of God. In fact, to Platonists, God is pure "Idea," while the task of the philosopher is to rise above human nature, as pure "Matter," and transcend to a higher level of "Idea."
The Metaphor of the Cave
Plato created the metaphor of the cave. In this story there are a group of individuals chained in a cave with a fire to their backs. They cannot turn their heads and may only gaze upon the shadows reflecting off the cave walls. The prisoners establish a primitive social order in which respect is granted to those who can correctly predict which (completely random) shape will next be projected onto the wall. One day someone escapes and, after a long and arduous struggle symbolising how difficult and uncomfortable it can be for Man to practice philosophy and profoundly reconsider the world around him, observes the real world. He returns to tell his companions of his amazing discovery only to be refuted and disbelieved by the cave dwellers, who cannot even comprehend what he is trying to tell them, and prefer to play their shadow games. Further hurting his credibility, having been in the light he finds it difficult to operate in the dim cave and appears clumsy, and completely unable to perceive the shadows - he has no social status and is ostracised as a literal 'outsider'. In this Plato illustrates the supposed futility of shallow living and paying heed to traditional and social convention, and proposes that Man must strive out on his own to gain true knowledge.