The tradition that takes seriously the question of what a fully just society would look like, and holds that the question is worth answering whether or not such a society could be realized — that a standard is needed before anything can be called an improvement.
Plato is its founding figure. The Republic asks not how to improve the city at hand but what justice itself would require if a society were built around it, and defends the procedure explicitly: a painter who depicts the most beautiful possible person is not refuted by the observation that no such person exists. More's Utopia, Kant's perpetual peace and the nineteenth-century utopian socialists all work in this register.
A note on the name, which is ambiguous in a way that causes real confusion. In international relations, idealism refers to something narrower and later — the Wilsonian program of collective security, international law and the League of Nations, defined against the realism this site treats separately. The two senses overlap in temperament but not in content, and a philosopher engaged in ideal theory need hold no particular view about the United Nations.
The contemporary form of the argument runs through Rawls, whose distinction between ideal theory — assuming full compliance and favorable conditions — and non-ideal theory made the methodological question explicit. Amartya Sen has pressed the strongest objection: that identifying the perfectly just society is neither necessary nor sufficient for ranking the actual alternatives in front of us, since knowing that a distant peak is highest tells you nothing about which of two nearer hills to climb. Charles Mills argued further that ideal theory functions ideologically, describing a world without the injustices that structure the real one and thereby making them harder to see. Defenders reply that every complaint about injustice smuggles in a standard, and that it is better to state it and defend it than to rely on it unexamined.