Platos Metaphysics and Epistemology:
Metaphysics: Theory of reality. What kinds of things exist? How many kinds of things are there? How are they related to one another? (E.g.: do minds exist? Numbers? God? Forms?)
Epistemology: Theory of knowledge. What is it to know something? What kinds of things can we know? What can justify our belief that something is true?
Plato holds that in a sense there are two separate worlds or realms; or, to put the point a little more tamely, that there are two very different kinds of things, ordinary physical objects and Forms. Here are some of the main differences between the two.
THE VISIBLE WORLD
THE INTELLIGIBLE WORLD
|The visible world consists of the things below the (main) line in the metaphor of the Divided Line: physical objects and their images, shadows, and reflections.||The intelligible world consists of the things above the (main) line in the metaphor of the Divided line: images and Forms.|
|Physical objects are constantly changing (in flux, to use the Heraclitean term). They are transient and ephemeral.||The Forms are unchanging and eternal.|
|Physical objects are less real than the Forms. Physical objects get what reality they have by their participation in the Forms.||The Forms are what really exists; the physical world is a kind of shadow or reflection of the world of the Forms.|
|We learn about physical objects empirically, by means of the senses: we look at them, taste them, listen to them, and so on. But none of the information we gain in this way is reliable or trustworthy: we dont have real knowledge of the visible world, just mere "opinion."||We learn about the Forms not by means of the sense but by means of Reason. We dont need to look at the Forms or listen to them; indeed we cannot do so. We figure out what they are by thinking about them. Empirical evidence is at best irrelevant, at worst misleading.|
|In a sense, though, knowledge of the forms also enables us to better understand the visible world. When we understand the Forms, we know what the visible world is a pale imitation of (as the person who returns to the cave better understands the shadows on the wall by virtue of knowing what they are shadows of).|
|The sun is what allows us to see physical objects.||The Good is what allows us to understand the Forms. (This is why the genuinely just person cant be a creep. Were just to the extent that our appetites and our reason are both properly developed and work together harmoniously. But when reason is developed, it makes us aware of the Good, and -- Plato thinks -- we cant know the Good without wanting to do it. So unless we do the good, we will inevitably be in inner turmoil.)|
|We have parts corresponding to the two worlds. Our physical bodies are a part of the visible world. Our bodies are responsible for our appetites. Our sense organs, by means of which we learn about the visible world, are also part of our physical body.||But theres also another part of us which links us with the eternal realm of the Forms, namely our soul (which for Plato is more or less identical with our reason). So one result of coming to learn about the Forms is that we will become less concerned with physical matters; we will be less governed by our appetites, and less reliant on our unreliable senses for knowledge.|