Pluralism is a term used in philosophy, referring to a worldview of multiplicity, often used in opposition to monism (the view that all is one) or dualism (the view that all is two). The term has different meanings in metaphysics, ontology, epistemology and logic. In metaphysics, it is the view that there are in fact many different substances in nature that constitute reality. In ontology, pluralism refers to different ways, kinds, or modes of being. For example, a topic in ontological pluralism is the comparison of the modes of existence of things like 'humans' and 'cars' with things like 'numbers' and some other concepts as they are used in science.

In epistemology, pluralism is the position that there is not one consistent means of approaching truths about the world, but rather many. Often this is associated with pragmatism, or conceptual, contextual, or cultural relativism. In the philosophy of science it may refer to the acceptance of co-existing scientific paradigms which though accurately describing their relevant domains are nonetheless incommensurable. In logic, pluralism is the relatively novel view that there is no one correct logic, or alternatively, that there is more than one correct logic. Such as using classical logic in most cases, but using paraconsistent logic to deal with certain paradoxes.

Metaphysical pluralism in philosophy is the multiplicity of metaphysical models of the structure and content of reality, both as it appears and as logic dictates that it might be, as is exhibited by the four related models in Plato's Republic and as developed in the contrast between phenomenalism and physicalism. Pluralism is in contrast to the concept of monism in metaphysics, while dualism is a limited form, a pluralism of exactly two models, structures, elements, or concepts. A distinction is made between the metaphysical identification of realms of reality and the more restricted sub-fields of ontological pluralism (that examines what exists in each of these realms) and epistemological pluralism (the methodology for establishing knowledge about these realms).

In ancient Greece, Empedocles wrote that they were fire, air, water and earth, although he used the word "root" rather than "element" (στοιχεῖον; stoicheion), which appeared later in Plato. From the association (φιλία; philia) and separation (νεῖκος; neikos) of these indestructible and unchangeable root elements, all things came to be in a fullness (πλήρωμα; pleroma) of ratio (λόγος; logos) and proportion (ἀνάλογος; analogos).

Similar to Empedocles, Anaxagoras was another Classical Greek philosopher with links to pluralism. His metaphysical system is centered around mechanically necessitated nous which governs, combines and diffuses the various "roots" of reality (known as homoioneroi). Unlike Empedocles' four "root elements" and similar to Democritus' multitude of atoms (yet not physical in nature), these homoioneroi are used by Anaxagoras to explain the multiplicity in reality and becoming. This pluralist theory of being influenced later thinkers such as Gottfried Wilhelm Leibniz's theory of monads and Julius Bahnsen's idea of will henades. The notion of a governing nous would also be used by Socrates and Plato, but they will assign it a more active and rational role in their philosophical systems.

Aristotle incorporated these elements, but his substance pluralism was not material in essence. His hylomorphic theory allowed him to maintain a reduced set of basic material elements as per the Milesians, while answering for the ever-changing flux of Heraclitus and the unchanging unity of Parmenides. In his Physics, due to the continuum of Zeno's paradoxes, as well as both logical and empirical considerations for natural science, he presented numerous arguments against the atomism of Leucippus and Democritus, who posited a basic duality of void and atoms. The atoms were an infinite variety of irreducibles, of all shapes and sizes, which randomly collide and mechanically hook together in the void, thus providing a reductive account of changeable figure, order and position as aggregates of the unchangeable atoms.

The topic of ontological pluralism discusses different ways, kinds, or modes of being. Recent attention in ontological pluralism is due to the work of Kris McDaniel, who defends ontological pluralism in a number of papers. The name for the doctrine is due to Jason Turner, who, following McDaniel, suggests that "In contemporary guise, it is the doctrine that a logically perspicuous description of reality will use multiple quantifiers which cannot be thought of as ranging over a single domain." "There are numbers, fictional characters, impossible things, and holes. But, we don't think these things all exist in the same sense as cars and human beings."

It is common to refer to a film, novel or otherwise fictitious or virtual narrative as not being 'real'. Thus, the characters in the film or novel are not real, where the 'real world' is the everyday world in which we live. However, some authors may argue that fiction informs our concept of reality, and so has some kind of reality.