Plato(428-348 B.C.) believed that there is a realm of abstract forms that is separate from the physical world. Everything in the world is an imperfect copy of one of these "pure" forms. Therefore, when we perceive the world, we are not seeing it as it truly is, but just crude, blurry copies of the forms that represent true reality. To make matters worse, these imperfect copies become distorted by our senses. Consequently, we can never come to know the true nature of the world through observation; the best we can do is form opinions based on faulty information. Plato illustrated his beliefs with many stories. One of the more famous of these is the "Allegory of the Cave" which you will be reading in this course.
For Plato, the best path to knowledge was through reflection, introspection, and rational thought. He believed that before the soul was implanted in the body it dwelled among the forms and had full and complete knowledge of them. Thus, we come into the world knowing the truth and through philosophical training we become able to rediscover it.(This view is called his "Reminiscence Theory" of knowledge.)
Neoplatonism, primarily through the writings of Plotinus(204-270 A.D.), was a revival of Plato's philosophy during the later stages of the Roman Empire. Plotinus developed the more mystical aspects of Plato's ideas and came very close to turning them into a religion rather than a philosophy. It was St. Augustine(345-430 A.D.) who later adapted Neoplatonism, adapted it to Christian beliefs, and put together a common core of Christian theology that would dictate the way Western peoples would try to understand the world for almost 1,000 years.
Aristotle(384-322 B.C.) rejected Plato's nativist beliefs and was a hard-nosed empiricist who examined nature directly rather than relying on introspection. He believed that reason was important, but also believed that sensory experience should be the raw material for it. He believed that to truly understand something one must carefully observe it and attempt to determine its four causes. These causes were the material cause(what is it made of?), the formal cause(what form does it take?), the efficient cause(what force shaped it?), and its final cause(why does it exist?).
While early Europeans were aware that Aristotle had existed, access to his work disappeared with the Fall of the Roman Empire and it was not rediscovered by Europe until contact with the Arab world increased following the crusades.