Reality refers to everything that exists or is real, as opposed to what is merely imaginary. This term also signifies the ontological status of things, that is, their existence. In physical terms, reality encompasses the entirety of the universe, both known and unknown. The nature of reality, its existence or being, is a central concern of ontology, a key subfield of metaphysics in Western philosophy.

Questions of ontology permeate various branches of philosophy, including the philosophy of science, religion, mathematics, and logic. Such questions explore whether only physical objects are real (Physicalism), if reality is fundamentally immaterial (Idealism), whether unobservable entities proposed by scientific theories exist, whether God exists, whether numbers and other abstract objects exist, and whether possible worlds exist.

Common parlance often equates reality with "perceptions, beliefs, and attitudes toward reality". For instance, "My reality is not your reality" might indicate agreement not to argue over deeply different concepts of what is real. Reality can be defined in connection to worldviews or parts of them: it is the sum of all things, structures (both actual and conceptual), events (past and present), and phenomena, whether observable or not.

Many theories of reality are shaped by ideas from physics, philosophy, sociology, literary criticism, and other fields. One such theory posits that there is no reality beyond the perceptions or beliefs we each have about reality, summed up in the popular statement, "Perception is reality", which indicates anti-realism—the belief that there is no objective reality.

Thomas Kuhn's The Structure of Scientific Revolutions (1962) expounds on the social construction of reality, emphasizing that the concepts of science and philosophy are often culturally and socially defined. The reality of everyday life is considered the most vital, as it requires our full awareness and attention.

Western philosophy approaches reality from two angles: the nature of reality itself, and the relationship between the mind (as well as language and culture) and reality. Ontology describes the most general categories of reality and their interrelations. This field is where a positive definition of reality would typically emerge. Philosophers often question whether existence (or reality) is a property of objects, with the prevailing view among analytic philosophers being that it is not.

Philosophical discussions of reality often concern the ways reality is, or is not, dependent upon mental and cultural factors. Realism supports the existence of a reality independent of any beliefs, perceptions, etc. Anti-realism, including idealism and phenomenalism, opposes this view.

The correspondence theory of knowledge asserts that true knowledge of reality represents accurate correlation of statements about and images of reality with the actual reality.

In metaphysics, the nature of being has always been a key topic. Perception also becomes crucial in the debate over whether we experience the real world itself (naïve realism or direct realism) or an internal perceptual copy of that world created by neural processes in our brain (indirect or representative realism).

In the philosophy of mathematics, the status of abstract entities, particularly numbers, is subject to debate. Platonic realism believes in the immaterial existence of numbers, while other forms of realism link mathematics with the concrete physical universe. There are anti-realist stances too, including formalism and fictionalism. Some theories are selectively realistic about some mathematical objects but not others.

An extreme form of mathematical realism, proposed by Max Tegmark, is the mathematical multiverse hypothesis, which posits that all structures that exist mathematically also exist physically. This theory suggests that every possible universe is real, and can be considered a form of Platonism or mathematical monism.

Universal truth:

he problem of universals grapples with whether abstract qualities like properties, characteristics, and relations, known as universals, genuinely exist. Realists argue for their existence, with Platonic realism and Aristotelian realism positing that universals are real entities, either independent of or dependent on particulars. On the other hand, anti-realist perspectives, such as nominalism and conceptualism, contest this view.

The philosophy of space and time considers the nature of these entities, with traditional realists advocating their existence independent of the human mind, while idealists question this. Kant posits that space and time are inherent notions that help structure our experience, while others suggest that time might be an illusion.

Various metaphysical theories offer differing views on the reality of the past, present, and future. While Presentism posits only the present as real, Eternalism (or block universe theory) assigns reality to all three temporal states, and the growing block universe theory holds only the past and present as real.

The notion of possible worlds arises from modal realism, which posits that all logically conceivable worlds are as real as the actual one. This idea interacts with theories of everything (TOEs), which seek to answer all critical questions in a coherent manner, providing a complete picture of the universe.

Phenomenological reality pertains to individual experiences and personal interpretations of events, a realm where spiritual experiences often occur. Phenomenology, a philosophical method, emphasizes understanding the structures of consciousness and the phenomena appearing in acts of consciousness.

Several skeptical hypotheses propose that reality is significantly different from our perception or that we can't prove otherwise. These include the "Brain in a vat" hypothesis, suggesting we might be disembodied brains fed with false sensory signals, and the "Dream argument", which asserts reality could be indistinguishable from a dream.

Jain philosophy outlines seven fundamental principles (tattvas) that make up reality, ranging from the soul (jīva) and the non-soul (ajīva) to liberation (moksha). In contrast, scientific realism proposes that the world depicted by science is the real world, irrespective of what we might perceive it to be.

Lastly, the notion of realism takes a different hue in physics, differing from metaphysical realism. While both agree on the mind-independent nature of reality, quantum mechanics challenges the traditional concept of local realism, especially with phenomena like quantum entanglement. This leads to discussions on counterfactual definiteness, asserting the definiteness of results for unperformed measurements.

The observer's role in quantum mechanics has been the topic of philosophical discussions. The founders of quantum mechanics, including Wolfgang Pauli and Werner Heisenberg, believed that the observer was responsible for wave function collapse. However, Albert Einstein critiqued this perspective as mystical and anti-scientific. Bohr, another proponent of quantum mechanics, never established a clear boundary between quantum and classical objects, viewing it as more of a philosophical question. Eugene Wigner, on the other hand, suggested that the consciousness of the observer is the line of demarcation, precipitating the collapse of the wave function, a view commonly known as “consciousness causes collapse”.

The concept of a multiverse, first proposed by William James in 1895, posits the existence of multiple potential universes, all of which together constitute everything in existence. The nature, structure, and interrelationships among these universes depend on the specific multiverse hypothesis under consideration. In the many-worlds interpretation of quantum mechanics, it is suggested that every possible quantum outcome happens in at least one universe.

The theory of everything (TOE) aims to explain and link all known physical phenomena and predict the outcomes of any theoretically possible experiment. However, unifying general relativity and quantum mechanics, two essential elements for a TOE, has proven to be a significant challenge. Some current TOE candidates include string theory, M theory, and loop quantum gravity.

Virtual reality (VR) is a computer-simulated environment that can imitate real-world places and imaginary worlds. The Virtuality Continuum is a continuous scale encompassing all variations and combinations of real and virtual objects, ranging from complete virtuality to reality. The mixed reality area within this continuum includes augmented reality and augmented virtuality.

On the internet, "real life" (RL) or "in real life" (IRL) refers to life as it is lived outside of the online world. With the growing integration of the internet into everyday life, the distinction between online and real-life worlds might eventually be seen as outdated. Some online activities, such as sexual intrigues, have already transitioned into complete legitimacy and "reality."