Both idealism and realism, as philosophical terms, deal with the relationship between our minds and the world. Idealism is the view that things exist only as ideas, with no reality as material objects outside of the mind. Realism is the view that objects exist in themselves, independently of our consciousness of them.
Realism asserts two fundamental things about the world: first, that objects outside of our mind have existence; second, that objects outside of our minds are independent from our minds, that is, that facts about these objects are true or false regardless of our opinions or beliefs. Idealists reject the idea that objects are independent of our minds. As the British idealist philosopher George Berkeley claimed in The Principles of Human Knowledge, “all those bodies which compose the mighty frame of the world, have not any subsistence without a mind.”
Idealism was a major force in German philosophy from the 1780s to the 1840s. The philosopher Immanuel Kant developed the philosophical doctrine of transcendental idealism: Although material things exist in some form, human beings only experience the appearances of things, and remain separated from things in themselves. The philosophers Johann Gottlieb Fichte, Friedrich Wilhelm Joseph Schelling, and Georg Wilhelm Friedrich Hegel developed a more radical view called absolute idealism, which holds that things in themselves cannot exist, that an object has to exist in our consciousness if it is to exist at all.
British Idealism and Analytic Philosophy
In the 19th century, the school of absolute idealism dominated British philosophy. The British idealists held that the world as we saw it was an illusion, that what we perceived as a material world filled with discrete objects was actually an indivisible whole, the nature of which was spiritual or mental, immaterial. At the beginning of the 20th century, G. E. Moore and Bertrand Russell broke from the philosophy of absolute idealism and founded the school of analytic philosophy, which endorsed a “common sense” realism and a rejection of grand systems of philosophy in favor of a focus on narrowly defined philosophical problems.
Selective Realism, Selective Idealism
Although there are a number of philosophies that apply realism or idealism broadly, it is possible to hold realist or idealist positions on a long list of philosophical subjects — ethics, aesthetics, causality, science, mathematics and semantics, among others — without taking a realist or idealist position on everything. For example, to say that one is a mathematical realist means that one believes that mathematical concepts exist in some form independent of human thought, but this does not require one to be a realist regarding ethical norms or aesthetic values.