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"World 3" redirects here. For the grouping of countries known as the "third world", see Third world. For the Maoist theory, see Three Worlds Theory. For the system dynamics model used in The Limits to Growth, see World3.

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Popper's three worlds is a theory developed by the British philosopher Karl Popper. It involves three interacting worlds, called World 1, World 2, and World 3. He considered these worlds to be real only to the extent that they allowed an epistemically relevant description of the evolution of the physical world and of life. The main objective of Popper was to defend, through World 3, the notion of objective knowledge against psychologism. In this theory, the methodological rules as well as the logical content of science belong to World 3.

Objective knowledge and psychologism

Already, in his central work in philosophy of science, The Logic of Scientific Discovery, Popper rejected psychologism, in which knowledge is viewed as a belief, a mental state, within individual minds, with no independent reality. He appreciated the scientific value of analyzing mental states from a biological or psychological perspective, but considered it uninteresting in his epistemology. In particular, he was much impressed at the time by Bolzano's view on objective knowledge. The idea of Popper's objective knowledge is based on the fact that, although the notion of "truth" (in the informal sense of correspondence with reality) is a guiding objective, knowledge is at no point evaluated as true or false in this sense, because Popper accepted the theses of Duhem and Hume, who asserted that this was impossible. Rather, knowledge is considered an objective resource for human beings, just as birds' nests, spiders' webs, and so on, are resources for animals. Therefore, the question of how to reject or accept knowledge in this philosophy is not intended to receive a rigorous answer. A methodology is proposed for that purpose and considered part of objective knowledge, but Popper asserts that it can hardly be rigorous. This objective knowledge, which belongs to World 3, is evaluated for its usefulness in its applications through critical discussions. This contrasts with the notion of belief, which must also be true, even justified, to qualify as knowledge.

Justificationism and objective knowledge

At the time Popper introduced his philosophy, the need to distinguish science from non-science through justification was a central theme of the Vienna Circle, Russell, Lakatos, and many others. Over the years, many approaches were tried and abandoned, but this justationist research program was not. It only underwent a profound change of form in the second half of the 20th century, around the time Gettier presented two simple counterexamples against the notion of knowledge as justified true belief. The problem of justifying our knowledge, especially scientific knowledge as attempted by the Vienna Circle, Russell, Popper, etc., has then shifted to the problem of defining knowledge analytically in terms of its components: beliefs, truth, justification, and, possibly, other requirements.

Popper emphasized that even though, according to Duhem's thesis, deductive logic cannot justify a truth in the sense of an empirical correspondence with reality, as the justificationist research program attempted to do, it can nevertheless be an important element in the development of objective knowledge. More specifically, Popper and Thornton described how the bulk of scientific activity consists of using deductive logic to check the consistency of a theory, compare theories, check their empirical validity (falsifiability), and, most importantly, test a theory, which is possible when it is falsifiable. They emphasized that, even in testing, deductive logic is largely used. Thus, even though Popper's theory is based on objective knowledge, it meets an important expectation of the justificationist program, and does so in harmony with usual scientific activities.

A key aspect of this description of the scientific process is the distinction between its logical and methodological aspects. Popper emphasized this distinction. The methodological aspect consists of rules that guide the overall process, for example, whether one theory is being tested or another. These rules can hardly be rigorous. The criterion of falsifiability belongs to the logical aspect and can be applied rigorously. Both aspects of science belong to World 3.

The three worlds

The numbering of the three worlds reflects their temporal order of emergence, each realm emerging as a product of the development of previous realms. A one-word description of each realm is that World 1 is the material realm, World 2 is the mental realm, and World 3 is the cultural realm.

Popper's theory is evolutionary. Popper maintains that at the inception of the universe there was only a "World 1", a realm where everything consisted of physical states and processes. A "World 2" of mental life later emerged as a product of biological evolution. Subsequently a "World 3" of cultural objects emerged as a product of evolution of the human "World 2". Popper was a strong advocate of a theory of emergence in which each world is not predetermined by previous ones. Against this, Popper argues that we should instead see the universe as "creative" and non deterministic, and as having given rise to genuinely new levels or realms - like biological life, "World 2" and "World 3". The three worlds may be understood, in terms of this evolutionary framework, as containing three categories of entity:

• World 1: the realm of states and processes as studied by the natural sciences. These include the states and processes that we seek to explain by physics and by chemistry, and also those states and processes that subsequently emerge with life and which we seek to explain by biology.

• World 2: the realm of mental states and processes. These include sensations and thoughts, and include both conscious and unconscious mental states and processes. World 2 includes all animal as well as human mental experience. These mental states and processes only emerge as a product of biological activity by living organisms, and so only emerged subsequent to the emergence of living organisms within World 1. Mental states and processes are the products of evolutionary developments in the World 1 of animal brains and nervous systems, but constitute a new realm of World 2 that co-evolved by its interaction with the World 1 of brains and nervous systems.

• World 3: the realm of the 'products of thought' when considered as objects in their own right. These products emerge from human "World 2" activity, but when considered as World 3 objects in their own right they have rebound effects on human World 2 thought processes. Through these rebound effects, World 3 'objects' may - via World 2-based human action on World 1 - have an indirect but powerful effect on World 1. In Popper's view, World 3 'objects' encompass a very wide range of entities, from scientific theories to works of art, from laws to institutions.

More on world 3

Popper says that his world 3 has much in common with Plato's theory of Forms or Ideas. But, world 3 is not to be conceived as a Platonic realm, because unlike the Platonic world of forms, which is non changing and exists independently of human beings, Popper's world 3 is created by human beings and is not fixed. It corresponds to the current state of our knowledge and culture.

Popper makes two key claims regarding the role of World 3 in the known universe. First, Popper argues that, despite the many continuities and correspondences between the human and animal World 2, (1) only humans consider their mental products as objects in their own right in a World 3 sense and (2) only humans have access to World 3 objects. Second, World 3 has no direct effect on World 1 but only affects World 1 as mediated by the human World 2. For example, a theory of nuclear reactions will never of itself cause a nuclear reactor to be built, yet the existence of a nuclear reactor is not the result of a purely World 1 process but is the eventual product of complex interactions between particular World 3 theories and human World 2 mental activity, and also particular World 2 and World 1 mind-brain-body interactions, leading to particular World 1 human actions (to construct a nuclear reactor) only made feasible by this complex set of interactions.

The interaction of world 1 and world 2

The theory of interaction between world 1 and world 2 is an alternative theory to Cartesian dualism, which is based on the theory that the universe is composed of two essential substances: res cogitans and res extensa. Popperian cosmology rejects this essentialism, but maintains the common sense view that physical and mental states exist, and they interact.

The interaction of world 2 and world 3

The interaction of world 2 and world 3 is based on the theory that world 3 is partially autonomous. For example, the development of scientific theories in world 3 leads to unintended consequences, in that problems and contradictions are discovered by world 2. Another example is that the process of learning causes world 3 to change world 2.

The interaction of world 3 and world 1

The world 3 objects are embodied in world 1. For example, the intrinsic value of Hamlet as a world 3 object is embodied many times in world 1. But, this representation of an object of world 3 in world 1 is not considered an interaction in Popper's view. Instead, for Popper, because world 3 is a world of abstractions, it can only interact with world 1 through world 2.

See also

• Collective consciousness
• Eccles' philosophy
• George Berkley
• Subjective idealism
• Four worlds
• Logical Investigations
• Platonic realm
• Third Realm (Frege)
• Trichotomy (philosophy)

References

Works cited

• Eccles, J. C. (1970). Facing Reality: Philosophical Adventures by a Brain Scientist. Springer Science & Business Media. ISBN 978-1-4757-3997-8.
• Freeman, Eugene; Skolimowski, Henryk (1974). "Peirce and Popper—Similarities and Differences". In Schilpp, Paul Arthur (ed.). The Philosophy of Karl Popper. Vol. II. Illinois: Open Court. pp. 509–519. ISBN 0-87548-142-6. OCLC 2580491.
• Heller, Michael (2011). Philosophy in Science: An Historical Introduction. Springer.
• Le Morvan, Pierre (2017). "Knowledge before Gettier". British Journal for the History of Philosophy. 25 (6): 1216–1238.
• Niiniluoto, Ilkka (2006). "World 3: A Critical Defence". In Jarvie, Ian; Milford, Karl; Miller, David (eds.). Karl Popper: Metaphysics and epistemology. Ashgate.
• Popper, Karl (1968) [reprinted in Popper 1972, chap. 3.]. Epistemology Without a Knowing Subject. Third International Congress for Logic, Methodology and Philosophy of Science, 25 August 1967. Amsterdam.
• Popper, Karl (1972). Objective Knowledge: An Evolutionary Approach.
• Popper, Karl R. (1994) [Based on his Emory University lectures 1969]. Knowledge and the body-mind problem : in defence of interaction. Mark Amadeus Notturno. London: Routledge. ISBN 0-415-11504-3. OCLC 30318882.
• Skolimowski, Henryk (1974). "Karl Popper and the Objectivity of Scientific Knowledge". In Schilpp, Paul Arthur (ed.). The Philosophy of Karl Popper. Vol. II. Illinois: Open Court. pp. 483–508. ISBN 0-87548-142-6. OCLC 2580491.
• Wagner, Pierre (2025-08-05). "Introduction". In Gallimard (ed.). Les Philosophes et la science (in French). pp. 9–65. Retrieved 2025-08-05.

Further reading

• Popper, Karl; Eccles, John C. (1977). The Self and Its Brain: An Argument for Interactionism.
• Popper, Karl (7 April 1978). Three Worlds by Karl Popper (PDF). The Tanner Lecture on Human Values. Talk delivered at The University of Michigan.
• Popper, Karl (1982). The Open Universe: An Argument for Indeterminism.
• Popper, Karl (1994b). Knowledge and the Body-Mind Problem.
• Popper, Karl (1994c) [1945]. The Open Society and Its Enemies. Routledge 2012, Princeton University Press 2013.