There is a use of the term “Realism” that is associated with a very special philosophical position that I consider a sub-element of Idealism. This group was primarily a medieval opposition to nominalism. But today it is used primarily to stress the importance of the particular as opposed to the universal. In other words, particular things exist and the universals we think of as concepts, mathematical relationships and abstractions “exist” only in the sense that people think those abstractions or concepts. Apart from our thinking those ideas, they only exist as particular things. We predicate abstract concepts to the subjects of our statements. In this sense, Aristotle is a Realist and is opposed to the Idealism of Plato. Also, in this sense, Empiricism is Realist and Rationalism is Idealist.
The scientific method may be considered the main way of knowing things from the Realist perspective. Mathematics may be considered the main way of knowing things from the Idealist perspective.
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