Berny Belvedere in Arc:
Ideal theory, as its name implies, is all about specifying what the perfect society would look like. But the goal isn’t purely academic; the point of ideal theory isn’t simply to satisfy an intellectual curiosity. Rather, conceptualizing the perfect society can supply us with a target to aim for.
If we can draw the outlines of the Good City, we can pattern our society after it. We can set sail for it.
Non-ideal theory sees this approach as hopelessly detached from reality. Since society is characterized by non-ideal circumstances — or, to put it more negatively (and accurately): since society is beset by far-from-ideal realities — and since some of these circumstances appear to be intractable, it makes little sense to discuss social justice within the rarefied air of ideal theory.
According to this view, doing ideal theory is almost like talking about what heaven might be like, which is interesting but not relevant to present concerns. An unbridgeable gap exists between the two worlds, and focusing on the happy afterlife one is an abdication of intellectual responsibility.
More here.