Reality, in everyday usage, means "the state of things as they actually exist". The term reality, in its widest sense, includes everything that is, whether or not it is observable or comprehensible. Reality in this sense may include both being and nothingness, whereas existence is often restricted to being (compare with nature). In other words, "reality", as a philosophical category, includes the formal concept of "nothingness" and articulations and combinations of it with other concepts (those possessing extension in physical objects or processes for example).
In the strict sense of western philosophy, there are levels or gradation to the nature and conception of reality. These levels include, from the most subjective to the most rigorous: phenomenological reality, truth, fact, and axiom.
On a much broader and more subjective level, the private experiences, curiosity, inquiry, and selectivity involved in the personal interpretation of an event shapes reality as seen by one and only one individual and hence is called phenomenological. This form of reality might be common to others as well, but at times could also be so unique to oneself as to be never experienced or agreed upon by any one else. Much of the kind of experience deemed spiritual occurs on this level of reality. From a phenomenological perspective, reality is that which is phenomenally real and unreality is nonexistent. Individual perception can be based upon an individual's personality, focus and style of attribution, causing him or her to see only what he or she wants to see or believes to be true.
For realists, the world is a set of definite facts, which obtain independently of humans ("The world is all that is the case" — Tractatus Logico-Philosophicus), and these facts are the final arbiter of truth. Michael Dummett expresses this in terms of the principle of bivalence: Lady Macbeth had three children or she did not; a tree falls or it does not. A statement will be true if it corresponds to these facts — even if the correspondence cannot be established. Thus the dispute between the realist and anti-realist conception of truth hinges on reactions to the epistemic accessibility (knowability, graspability) of facts.
This view of reality is well expressed by Philip K. Dick's statement that "Reality is that which, when you stop believing in it, doesn't go away."
In philosophy, reality is contrasted with nonexistence (penguins do exist; so they are real) and mere possibility (a mountain made of gold is merely possible, but is not known to be real—that is, actual rather than possible—unless one is discovered). Sometimes philosophers speak as though reality is contrasted with existence itself, though ordinary language and many other philosophers would treat these as synonyms. They have in mind the notion that there is a kind of reality — a mental or intentional reality, perhaps — that imaginary objects, such as the aforementioned golden mountain, have. Alexius Meinong is famous, or infamous, for holding that such things have so-called subsistence, and thus a kind of reality, even while they do not actually exist. Most philosophers find the very notion of "subsistence" mysterious and unnecessary, and one of the shibboleths and starting points of 20th century analytic philosophy has been the forceful rejection of the notion of subsistence — of "real" but nonexistent objects.
Some schools of Buddhism hold that reality is something void of description, the formless which forms all illusions or maya. Buddhists hold that we can only discuss objects which are not reality itself and that nothing can be said of reality which is true in any absolute sense. Discussions of a permanent self are necessarily about the reality of self which cannot be pointed to nor described in any way. Similar is the Taoist saying, that the Tao that can be named is not the true Tao, or way.
It is worth saying at this point that many philosophers are not content with saying merely what reality is not — some of them have positive theories of what broad categories of objects are real, in addition. See ontology as well as philosophical realism; these topics are also briefly treated below.
One of the fundamental issues in ethics is called the is-ought problem, and it can be formulated as follows: "Given our knowledge of the way the world 'is,' how can we know the way the world 'ought to be'?" Most ethical views hold that the world we live in (the real world) is not ideal — and, as such, there is room for improvement.
In the arts there was a broad movement beginning in the 19th century, realism (which led to naturalism), which sought to portray characters, scenes, and so forth, realistically. This was in contrast and reaction to romanticism, which portrayed their subjects idealistically. Commentary about these artistic movements is sometimes put in terms of the contrast between the real and the ideal: on the one hand, the average, ordinary, and natural, and on the other, the superlative, extraordinary, improbable, and sometimes even supernatural. Obviously, when speaking in this sense, "real" (or "realistic") does not have the same meaning as it does when, for example, a philosopher uses the term to distinguish, simply, what exists from what does not exist.
In the arts, and also in ordinary life, the notion of reality (or realism) is also often contrasted with illusion. A painting that precisely indicates the visually-appearing shape of a depicted object is said to be realistic in that respect; one that distorts features, as Pablo Picasso's paintings are famous for doing, are said to be unrealistic, and thus some observers will say that they are "not real." But there are also tendencies in the visual arts toward so-called realism and more recently photorealism that invite a different sort of contrast with the real. Trompe-l'œil (French, "fool the eye") paintings render their subjects so "realistically" that the casual observer might temporarily be deceived into thinking that he is seeing something, indeed, real — but in fact, it is merely an illusion, and an intentional one at that.
In psychiatry, reality, or rather the idea of being in touch with reality, is integral to the notion of schizophrenia, which has often been defined in part by reference to being "out of touch" with reality. The schizophrenic is said to have hallucinations and delusions which concern people and events that are not "real." However, there is controversy over what is considered "out of touch with reality," particularly due to the noticeable comparison of the process of forcibly institutionalising individuals for expressing their beliefs in society to reality enforcement. The practice's possible covert use as a political tool can perhaps be illustrated by the 18th century psychiatric sentences in the U.S. of black slaves for 'crazily' attempting to escape. See also anti-psychiatry and one of its prominent figures, the psychiatrist Thomas Szasz.
In each of these cases, discussions of reality, or what counts as "real," take on quite different casts; indeed, what we say about reality often depends on what we say it is not.
A common colloquial usage would have "reality" mean "perceptions, beliefs, and attitudes toward reality," as in "My reality is not your reality." This is often used just as a colloquialism indicating that the parties to a conversation agree, or should agree, not to quibble over deeply different conceptions of what is real. For example, in a religious discussion between friends, one might say (attempting humor), "You might disagree, but in my reality, everyone goes to heaven."
Reality can be defined in a way that links it to worldviews or parts of them (conceptual frameworks): Reality is the totality of all things, structures (actual and conceptual), events (past and present) and phenomena, whether observable or not. It is what a worldview (whether it be based on individual or shared human experience) ultimately attempts to describe or map.
Certain ideas from physics, philosophy, sociology, literary criticism, and other fields shape various theories of reality. One such belief is that there simply and literally is no reality beyond the perceptions or beliefs we each have about reality. Such attitudes are summarized in the popular statement, "Perception is reality" or "Life is how you perceive reality" or "reality is what you can get away with" (Robert Anton Wilson), and they indicate anti-realism - that is, the view that there is no objective reality, whether acknowledged explicitly or not. These topics will be discussed in greater detail below.
On the one hand, ontology is the study of being, and the central topic of the field is couched, variously, in terms of being, existence, "what is", and reality. The task in ontology is to describe the most general categories of reality and how they are interrelated. If — what is rarely done — a philosopher wanted to proffer a positive definition of the concept "reality", it would be done under this heading. As explained above, some philosophers draw a distinction between reality and existence. In fact, many analytic philosophers today tend to avoid the term "real" and "reality" in discussing ontological issues. But for those who would treat "is real" the same way they treat "exists", one of the leading questions of analytic philosophy has been whether existence (or reality) is a property of objects. It has been widely held by analytic philosophers that it is not a property at all, though this view has lost some ground in recent decades.
On the other hand, particularly in discussions of objectivity that have feet in both metaphysics and epistemology, philosophical discussions of "reality" often concern the ways in which reality is, or is not, in some way dependent upon (or, to use fashionable jargon, "constructed" out of) mental and cultural factors such as perceptions, beliefs, and other mental states, as well as cultural artifacts, such as religions and political movements, on up to the vague notion of a common cultural world view, or Weltanschauung.
The view that there is a reality independent of any beliefs, perceptions, etc., is called realism. More specifically, philosophers are given to speaking about "realism about" this and that, such as realism about universals or realism about the external world. Generally, where one can identify any class of object the existence or essential characteristics of which is said not to depend on perceptions, beliefs, language, or any other human artifact, one can speak of "realism about" that object.
One can also speak of anti-realism about the same objects. Anti-realism is the latest in a long series of terms for views opposed to realism. Perhaps the first was idealism, so called because reality was said to be in the mind, or a product of our ideas. Berkeleyan idealism is the view, propounded by the Irish empiricist George Berkeley, that the objects of perception are actually ideas in the mind. On this view, one might be tempted to say that reality is a "mental construct"; this is not quite accurate, however, since on Berkeley's view perceptual ideas are created and coordinated by God. By the 20th century, views similar to Berkeley's were called phenomenalism. Phenomenalism differs from Berkeleyan idealism primarily in that Berkeley believed that minds, or souls, are not merely ideas nor made up of ideas, whereas varieties of phenomenalism, such as that advocated by Russell, tended to go farther to say that the mind itself is merely a collection of perceptions, memories, etc., and that there is no mind or soul over and above such mental events. Finally, anti-realism became a fashionable term for any view which held that the existence of some object depends upon the mind or cultural artifacts. The view that the so-called external world is really merely a social, or cultural, artifact, called social constructionism, is one variety of anti-realism. Cultural relativism is the view that social issues such as morality are not absolute, but at least partially cultural artifact.
A Correspondence theory of knowledge about what exists claims that "true" knowledge of reality represents accurate correspondence of statements about and images of reality with the actual reality that the statements or images are attempting to represent. For example, the scientific method can verify that a statement is true based on the observable evidence that a thing exists. Many humans can point to the Rocky Mountains and say that this mountain range exists, and continues to exist even if no one is observing it or making statements about it. However, there is nothing that we can observe and name, and then say that it will exist forever. Eternal beings, if they exist, would need to be described by some method other than scientific.
Quantum mechanics (QM) has kept physicists and philosophers in debate on the nature of reality since its invention. QM states that prior to observation, nothing can be said about a physical system other than a probability function which seems to be definable to a degree by assumptions about the system's elements. With observation a system's probability wave function will collapse into a precise quantity which is observable by the means of measuring the device applied. Heisenberg's uncertainty principle states that there are certain measurements that reduce the accuracy of other measurements of the same system. Primarily, one cannot measure the location and velocity of sub-atomic elements such as an electron precisely because the more one looks for the former the less accuracy one can achieve for the latter. This imprecision introduces an uncertainty into the overall state of the system and the necessity of a choice on the part of the one making the measurement, namely which aspect will he find accurately at the cost of the other. This decision on the part of the measurer has created no small problem for objectivists who insist that at its core reality is objectively present whether anyone notices or not. Several experiments such as the double-slit experiment, and tests of Bell's theorem and the CHSH inequality have confirmed that the simple act of observing does impact the system's state in a noticeable way; since the detector itself has to be changed to detect anything at all, there is necessarily a change in the observed particle because of quantum entanglement. But also the state of correlated particles which have not been measured appears to be affected. Even the notion of cause and effect is brought into question in the quantum world where irreducible randomness cannot currently be avoided as a basic assumption. In theory large numbers of random quantum elements seen as a group from a very great distance can seem like cause and effect which is why our level of experience appears to function almost completely deterministically.
It has led some such as Amit Goswami, a theoretical nuclear physicist and member of The University of Oregon, to assume that there is no reality existing, independent of our own consciousness as observer. However, there is no clear evidence that human consciousness has any special role to play beyond the influence of instrument-settings on result. These phenomena can also be given the more cautious interpretation that quantum systems do contain properties, but not properties directly corresponding to measurements performed on the system by macroscopic instruments.