A tenet of Plato's worldview was the concept that everything in the observable world was defined by the degree it partook in its 'ideal' self. Each thing owed its 'thing-ness' to some ultimate form - a statue was beautiful in that it partook to some degree in the idea of 'beauty'. This was referred to as Plato's theory of forms, and it sought to address the Problem of Universals. Does a property exist outside of the things that exhibit said property, i.e. universally? Plato argued that it did, as did other philosophers of his time.
What struck me about this theory, when I came across it, was its relation to the nature of Greek (and by extension Roman) pantheism. Here we have a pantheon of Gods, each assigned an immutable attribute or set of attributes. In the context of Plato's theory of forms, these ultimate personifications make perfect sense. The degree to which X thing partakes in Y ultimate attribute is the degree to which it partakes in some God-nature. Indeed this polytheistic tradition of gods exhibiting ultimate universal Forms springs up all over the place - it reminds me of the meso-American pantheon, and indeed the dozens of Saints that linger in modern day Catholicism.
The theory of forms is a rebuttal to monotheism - as Plato discusses these particular attributes, a singular Deity doesn't correlate well with myriad, but discrete, ultimate forms.
I can't say that I agree with much of it as a modern day relativist (aren't we all?). If all tables across the world partake in some form of an ultimate Table, how do you define that Table, and how do you distinguish it from something like it? Isn't it an assignment of meaning (in words) through connections in our own minds? It's a question of language more than anything.