In science, a theory is repeatedly tested and once shown to be reliable, accepted as knowledge rather than idea. It must be substantiated in a way that incorporates known scientific laws, facts, and hypotheses. Strictly speaking, a scientific theory can only be proven wrong, never completely right, because all possible circumstances are not testable. However, theories that are self-consistent and consistent with other knowledge become accepted. The theory of evolution, for example, maintains the theoretical idea that evolution operates by natural selection, which removes unfit organisms from a changing environment, leaving an evolved gene pool.
In the social sciences, theories develop based on evidence collected over time about human social behavior, culture and history. They predict social outcomes and may interpret and shape societies and include a strong philosophical component and may be limited by cultural and historical context. Social utilitarianism, for example, holds that moral choices should be made based on the balance of positive over negative consequences for the society as a whole. One of its theoretical ideas is that such moral evaluations can be made impartially.
Political theory is often pinned on preexisting ideas about political realities and moralities. Theoretical ideas in political theory may or may not have much evidence behind them. For example, Marxism is a social theory based on observations about capitalism and human culture. One of its theoretical ideas, historical materialism, holds that history is about struggles between different social classes. In the context of his theory, this idea is unquestioned, making it purely theoretical.
Aesthetic theory refers to ideas about the nature of beauty, art, and artistic meaning. They vary and are not necessarily meant to be universal. Aesthetic theories reflect the culture and time that create them, and many of the theoretical ideas that drive an aesthetic theory are not relevant outside of context. For example, one premise behind modern aesthetics states that art is non-utilitarian and not to be understood as a tool for social engineering, commercial gain or political statement. Aesthetic value resides instead only in its inherent beauty and meaning.