Plato's Republic is heavily influenced by geometry, the big intellectual innovation of the time. Of course the major weirdness of geometry is that you can design a real object like a building, and base that design on an unreal thing, like a square or a circle or a triangle, and it'll work. So the big disconnect is that the logic of squares and circles and triangles is powerful and effective and works in a pragmatic sense, and yet no true perfect squares, circles, or triangles exist in real life.
So the solution to this is to posit that outside the universe there dwells a pure realm of math, where the real squares and triangles et cetera exist, and to say that our own world is just a shadowy reflection of that pure greatness.
And this philosophy has all kinds of consequences, some of them markedly non-beneficial, but that's another subject. The reason I bring all that up is that's Java.
Anybody who comes to Ruby from Java will sooner or later encounter confusion about the assigning of instance variables to classes. You would think those instance variables would then become available to instances of the class, as class variables; in fact, from the point of view of an instance of a given class, an instance variable assigned to a class just disappears into a cloud of mystery.
The solution is that an instance variable assigned to a class isn't actually assigned to a class at all; it's assigned to a Class. Ruby's object model is very closely based on Smalltalk's, where classes themselves are objects in the system, available for real-time modification by the programmer. This means that if you assign an instance variable to a class, it's available to methods within the class itself, but of course not to instances of the class, because in Ruby, a class isn't an abstract Platonic definition that sits outside the world itself. It's right there in the world with everything else, and an instance of it is just something which implements the definition it provides.
See the thread on the ruby-talk mailing list about instance variables and ActiveRecord::Base for a more detailed discussion (and a nice examination of the Rails source code).