Realism (in philosophy) about a given object is the view that this object exists in reality independently of our conceptual scheme. In philosophical terms, these objects are ontologically independent of someone's conceptual scheme, perceptions, linguistic practices, beliefs, etc.
Realism can be applied to many philosophically interesting objects and phenomena: other minds, the past or the future, universals, mathematical entities (such as natural numbers), moral categories, the physical world, and thought.
Realism can also be a view about the nature of reality in general, where it claims that the world exists independent of the mind, as opposed to anti-realist views (like some forms of skepticism and solipsism, which deny the existence of a mind-independent world). Philosophers who profess realism often claim that truth consists in a correspondence between cognitive representations and reality.
Realists tend to believe that whatever we believe now is only an approximation of reality but that the accuracy and fullness of understanding can be improved. In some contexts, realism is contrasted with idealism. Today it is more usually contrasted with anti-realism, for example in the philosophy of science.
Platonic realism is a philosophical term usually used to refer to the idea of realism regarding the existence of universals or abstract objects after the Greek philosopher Plato (c. 427–c. 347 BC), a student of Socrates. As universals were considered by Plato to be ideal forms, this stance is confusingly also called Platonic idealism. This should not be confused with Idealism, as presented by philosophers such as George Berkeley: as Platonic abstractions are not spatial, temporal, or mental, they are not compatible with the latter Idealism's emphasis on mental existence. Plato's Forms include numbers and geometrical figures, making them a theory of mathematical realism; they also include the Form of the Good, making them in addition a theory of ethical realism.
Medieval realism developed out of debates over the problem of universals. Universals are terms or properties that can be applied to many things, such as "red", "beauty", "five", or "dog". Realism in this context, contrasted with conceptualism and nominalism, holds that such universals really exist, independently and somehow prior to the world. Moderate Realism holds that they exist, but only insofar as they are instantiated in specific things; they do not exist separately from the specific thing. Conceptualism holds that they exist, but only in the mind, while nominalism holds that universals do not "exist" at all but are no more than words (flatus vocis) that describe specific objects.
Scottish Common Sense Realism is a school of philosophy that sought to defend naive realism against philosophical paradox and scepticism, arguing that matters of common sense are within the reach of common understanding and that common-sense beliefs even govern the lives and thoughts of those who hold non-commonsensical beliefs. It originated in the ideas of the most prominent members of the Scottish School of Common Sense, Thomas Reid, Adam Ferguson and Dugald Stewart, during the 18th century Scottish Enlightenment and flourished in the late 18th and early 19th centuries in Scotland and America.
Its roots can be found in responses to such philosophers as John Locke, George Berkeley and David Hume. The approach was a response to the "ideal system" that began with Descartes' concept of the limitations of sense experience and led Locke and Hume to a skepticism that called religion and the evidence of the senses equally into question. The common sense realists found skepticism to be absurd and so contrary to common experience that it had to be rejected. They taught that ordinary experiences provide intuitively certain assurance of the existence of the self, of real objects that could be seen and felt and of certain "first principles" upon which sound morality and religious beliefs could be established. Its basic principle was enunciated by its founder and greatest figure, Thomas Reid:
Naïve realism, also known as direct realism, is a philosophy of mind rooted in a common sense theory of perception that claims that the senses provide us with direct awareness of the external world. In contrast, some forms of idealism assert that no world exists apart from mind-dependent ideas and some forms of skepticism say we cannot trust our senses. The naive realist view is that objects have properties, such as texture, smell, taste and colour, that are usually perceived absolutely correctly. We perceive them as they really are.
Scientific realism is, at the most general level, the view that the world described by science is the real world, as it is, independent of what we might take it to be. Within philosophy of science, it is often framed as an answer to the question "how is the success of science to be explained?" The debate over what the success of science involves centers primarily on the status of unobservable entities apparently talked about by scientific theories. Generally, those who are scientific realists assert that one can make reliable claims about unobservables (viz., that they have the same ontological status) as observables. Analytical philosophers generally have a commitment to scientific realism, in the sense of regarding the scientific method as a reliable guide to the nature of reality. The main alternative to scientific realism is instrumentalism.
Realism in physics (especially quantum mechanics) is the claim that the world is in some sense mind-independent: that even if the results of a possible measurement do not pre-exist the act of measurement, that does not require that they are the creation of the observer (contrary to the "consciousness causes collapse" interpretation of quantum mechanics). That interpretation of quantum mechanics, on the other hand, states that the wave function is already the full description of reality. The different possible realities described by the wave function are equally true. The observer collapses the wave function into their own reality. One's reality can be mind-dependent under this interpretation of quantum mechanics.
Aesthetic realism (not to be confused with the Aesthetic Realism of Eli Siegel, the philosophy that sees reality as the making one of opposites) may mean the claim that there are mind-independent aesthetic facts, but in general discussions about art "realism" is a complex term that may have a number of different meanings.