If my understanding is correct, Plato's theory of forms was developed to counter the relativism of Sophists such as Protagoras. His theory does not seem to have modern followers, as far as I know, yet how do modern philosophers justify the existence of "The Good" for example? Or more generally, how do they oppose relativism?
Most contemporary philosophers accept that relativism is self-refuting.
By "relativism," in this sense, I mean what I take Plato to being arguing against in the dialogues Theaetetus and Sophist. It is committed to two views:
- There are no objective truths. By "objective truth" I mean a truth which one must believe, or else be rightly considered irrational. Similarly, we could understand a truth to be objective if a being — even an alien from another planet — would agree to it, were the alien to have knowledge and understanding of the same things. The mass of a pebble is an example. You can express it in English, Spanish, Korean, or hexadecimal notation, or (maybe?) Klingon, but it all translates to the same fact, or else isn't true. Relativism denies that there are any truths like that.
- There are, however, local truths, says the relativist, whose truth is merely relative to the culture or other group believing them. Relativists say about these latter kinds of truths that they are "true FOR X" where X is a culture or community, or even an individual. Relativists do not therefore deny the existence of truths, but understand all truths to be relative truths with currency only among a community that believes them.
This form of relativism, which we might call "general relativism" to separate it from "moral relativism," is generally considered self-refuting, and therefore untenable. It suggests that there are no truths that hold objectively, but it insists that the claims above are objective truths.
Consider, that is, what a relativist would have to say about relativism: "Relativism is true for me but maybe not for you," in which case we have no reason to accept it. Indeed, if the relativist is right, relativism can have no sway over us, because a thing can be made true simply by being believed, and made false simply by not being believed. Or the relativist could say "Relativism is objectively true," in which case Relativism simply contradicts itself.
Badiou, a French philosopher counters the modern form of sophistry (Postmodernism and other ills) by what might be seen as a modern form of Platos forms by steering philosophy towards a philosophy rooted in mathematics - he takes his starting point the axiomatisation of it within set theory - that is ZFC.
One might say he's resurrecting a grand narrative.
Philosophy for him is a point from which three 'truth procedures' are suspended from: Love, Science & Art. (Being an avowed communist of the Maoist kind - he is rigourously atheistic, but for the religously inclined philosopher one may suppose that Religion comes under Love).
Nina Powers, a British Philosopher (a follower of Badiou) takes a more modest view of philosophy and considers it empty of content (or taking a 'Buddhistic' view - lacking its own essence), and thus by only embracing one of those three conditions can it be said to have content or substance.