Contemporary philosophical realism, also referred to as metaphysical realism, is the belief in a reality that is completely ontologically independent of our conceptual schemes, linguistic practices, beliefs, etc. Philosophers who profess realism also typically believe that truth consists in a belief's correspondence to reality. We may speak of realism with respect to other minds, the past, the future, universals, mathematical entities (such as natural numbers), moral categories, the material world, or even thought.

Realists tend to believe that whatever we believe now is only an approximation of reality and that every new observation brings us closer to understanding reality.^[1] In recent times, debates concerning realism have become quite contentious due mostly in part to the influence of postmodernism. Realism is contrasted with anti-realism.

Debates about realism

Despite the seeming straightforwardness of the realist position, in the history of philosophy there has been continuous debate about what is real. In addition, there has been significant evolution in what is meant by the term "real".

Universals

The oldest use of the term comes from medieval interpretations and adaptations of Greek philosophy. In this medieval scholastic philosophy, however, "realism" meant something different -- indeed, in some ways almost opposite -- from what it means today. In medieval philosophy, realism is contrasted with "conceptualism" and "nominalism". The opposition of realism and nominalism developed out of debates over the problem of universals. Universals are terms or properties that can be applied to many things, rather than denoting a single specific individual--for example, red, beauty, five, or dog, as opposed to "Socrates" or "Athens". Realism in this context holds that universals really exist, independently and somehow prior to the world; it is associated with Plato. Conceptualism holds that they exist, but only insofar as they are instantiated in specific things; they do not exist separately. Nominalism holds that universals do not "exist" at all; they are no more than words we use to describe specific objects, they do not name anything. This particular dispute over realism is largely moot in contemporary philosophy, and has been for centuries.

Matter

In its Kantian sense, realism is contrasted with idealism'. In a contemporary sense, realism is contrasted with anti-realism, primarily in the philosophy of science.

In practice

Both these disputes are often carried out relative to some specific area: one might, for example, be a realist about physical matter but an anti-realist about ethics. The high necessity of specifying the area in which the claim is made has been increasingly acknowledged in recent years.

Increasingly these last disputes, too, are rejected as misleading, and some philosophers prefer to call the kind of realism espoused there "metaphysical realism," and eschew the whole debate in favour of simple "naturalism" or "natural realism", which is not so much a theory as the position that these debates are ill-conceived if not incoherent, and that there is no more to deciding what is really real than simply taking our words at face value.

Some realist philosophers prefer deflationary theories of truth to more traditional correspondence accounts.

Realism in logic and mathematics

Mathematical realism, like realism in general, holds that mathematical entities exist independently of the human mind. Thus humans do not invent mathematics, but rather discover it, and any other intelligent beings in the universe would presumably do the same. In this point of view, there is really one sort of mathematics that can be discovered: Triangles, for example, are real entities, not the creations of the human mind.

Many working mathematicians have been mathematical realists; they see themselves as discoverers of naturally occurring objects. Examples include Paul Erdős and Kurt Gödel. Gödel believed in an objective mathematical reality that could be perceived in a manner analogous to sense perception. Certain principles (e.g., for any two objects, there is a collection of objects consisting of precisely those two objects) could be directly seen to be true, but some conjectures, like the continuum hypothesis, might prove undecidable just on the basis of such principles. Gödel suggested that quasi-empirical methodology could be used to provide sufficient evidence to be able to reasonably assume such a conjecture.

Within realism, there are distinctions depending on what sort of existence one takes mathematical entities to have, and how we know about them.

Realism in physics

Realism in physics refers to the fact that any physical system must have it property defined, whether or not it is measured (or observed or not). However, Quantum Mechanics states it is not valid to speak that a system has some property unless that property is measured. This implies that quantum systems exhibit a non-local behaviour. Bell's theorem proved that every quantum theory must either violate local realism or counterfactual definiteness. Physics up to the 19th century was always implicitly and sometimes explicitly taken to be based on philosophical realism. With the advent of quantum mechanics in the 20th century, it was noted that it is no longer possible to adhere local realism — that is, to both the principle of locality (that distant objects cannot affect local objects), and a form of ontological realism implicit in classical physics. This has given rise to a contentious debate of the interpretation of quantum mechanics. Although locality and 'realism' are jointly false, it is possible to retain one of them. The majority of working physicists discard 'realism' in favor of locality, since non-locality is held to be contrary to relativity. The implications of this stance are rarely discussed outside of the microscopic domain. See, however, Schrödinger's cat for an illustration of the difficulties presented. It can also be argued that the 'realism' of physics is a much more specific notion than general philosophical realism.^[2]