What Are Karl Popper’s Three Worlds?

Karl Popper's Three Worlds Theory of reality recognizes a fundamental distinction between physical objects, subjective states, and the products of human creativity and inquiry.

May 30, 2023By Andres Felipe Barrero, MA Philosophy, MSc Philosophy, Ph.D. Candidate

karl popper three worlds


Anyone interested in the philosophy of science has, at some point, heard of Karl Popper. Karl Popper is frequently recognized for his idea of falsifiability presented in The Logic of Scientific Discovery in 1957. He reasoned that the criterion that demarcated science from pseudo-science was that the first could be falsified.


Nonetheless, his philosophy extends far beyond the concept of falsifiability, and even to the realm of political philosophy. A great example of the richness of his ideas is his take on the ontology of knowledge and culture, and his proposal to differentiate between three worlds. His theory of the three worlds is the core of the following paragraphs.


Overview of Karl Popper’s Three Worlds 

Karl Popper 1991
Karl Popper in 1991, via Britannica


Sir Karl Raimund Popper was born in 1902 in Vienna. He attended the University of Vienna in 1918, and by 1928 he earned a doctorate in psychology (Thornton, 2022). He would become one of the most influential philosophers of science and teach at renowned universities like the London School of Economics and the University of New Zealand.


His theory of the Three Worlds can be found in his book Objective Knowledge, published in 1972, but also in a conference given at the University of Michigan in 1978 in the context of The Tanner Lectures on Human Values. The thrust of his theory is that the products of human cognition and creativity —i.e., knowledge and culture— are objective, insofar, as they can endure without a knowing subject and have an independent ontological status. These objects are distinct from both the physical world and the subjective world. To grasp those claims, let us start by providing an overview of each world.

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“World 1“ is the one consisting of all physical bodies: plants, animals, stars, and stones, but also radiation and gravity. World 1 denotes all entities in their physical, chemical, and biological dimension. It should not be confused with material things, given that entities such as forces are not exactly material. Popper proposes that this first world could be subdivided into non-living and living things (1978, p. 143).


animal creation
The Animal Creation, 1875, Published by Currier & Ives, via the Met Museum


World 2 represents, on the other hand, a mental and psychological dimension. It is the world of pain, pleasure, thoughts, ideas, fears, and hopes. Our teeth are part of World 1, but a toothache is subjective because it is a matter of personal experience and thus belongs to World 2. Intriguingly, Popper is not exclusively referring to consciousness or human subjectivity. Subconscious experiences, such as dreams, are part of World 2; additionally, animal consciousness takes place in World 2.


It is already obvious that Popper is distancing himself from any reductionist ideas in which mental states can be explicated only in terms of World 1. Fear can be described chemically, but Popper claims that it is something more, i.e. an experience.


A useful example is that of the brain and the mind. For the Austrian philosopher, the brain is part of World 1, but the mind is something not reducible to its physical components. Researchers interested in understanding dreams have taken MRIs (magnetic resonance imaging) of people sleeping, and they can observe which segments of the brain activate. However, they have to rely on the subjective reports of people in order to know what they were dreaming about (Walker, 2017, pt. III).


Finally, Karl Popper refers to World 3 as “the world of objective contents of thought, especially of scientific and poetic thoughts and of works of art” (1979, p. 106). World 3 is constituted by languages, tales, myths, scientific conjectures, mathematical propositions, paintings, sculptures, and symphonies (Popper, 1978, p. 144). These are all products of the mind, but are identical not the mind itself.


The Third World

brain scan
MRI of a human brain, via the University of Oxford.


Argentinian writer Jorge Luis Borges once said that he imagines paradise to be a kind of library. Nobody would say that Borges is interested in the shape of the books or the texture of the pages. Even if these were to play a role in his imagination, the point of a library is that it contains thousands of ideas.


A book, for Popper, is both an object of World 1 and World 3. In terms of World 1, a book is composed of pages, a cover, and graphemes. Nonetheless, the graphemes are only linguistic expressions of ideas (World 3). Ideas are at the core of any book and can be translated into other linguistic codes. No matter how difficult some translations may be, especially in literature, there is something that remains invariant: the World 3. Popper describes the physical dimension of books as embodiments of World 3 objects. There are numerous types of embodiments.


Let us turn to music to deepen this point. Beethoven’s Fifth Symphony can be embodied in an array of ways: it can be in musical notation, in the performance of a pianist, and even in the memory traces on people’s brains (Popper, 1978, p. 146). These embodiments are also called physical realizations; they are the concrete forms of an abstract object: Beethoven’s Fifth Symphony.


Consequently, World 3 can be better depicted as the great library of human knowledge and culture. A key characteristic is that the ideas inhabiting World 3 can endure even in the absence of a knowing subject. In a thought experiment, Popper asserts that if all our machines and tools were to be destroyed, and all subjective knowledge forgotten, but our libraries and our ability to learn from them were left, we could rebuild civilization (In Gödert & Lepsky, 2019, p. 3)


austria admont library
Admont Abbey Library, Austria. Photograph by Jorge Royan, via Wikimedia Commons.


Scientific theories and mathematical statements are also part of  Word 3. A scientific claim can be expressed in numerous ways, but its content is the same. Strictly speaking, the mathematical system we use is another type of language, so there is no reason to think that it is the only one. The content of the proposition remains invariant for scientific statements.


Take, for instance, the Pythagorean theorem which states that for any right triangle, the following is true:


 a²+b²= c² 


where c is the hypothenuse. Of course, human ingenuity could have devised another way of encoding this proposition, another embodiment or physical realization. The above statement is not subjective because it can be criticized and demonstrated by every human being. Popper sees this as an additional reason to believe that mathematical propositions are objective knowledge. To this day, there are 371 proofs of the Pythagorean theorem: some are algebraic, while others are geometric; they are all embodiments of the same object in World 3.


Popper’s Intellectual Contenders 

Pythagoras knapp
Pythagoras, painted by Augustus Knapp in 1926, via Wikimedia Commons.


With his proposal, Popper is challenging monist and dualist conceptions of reality. He declares himself a “pluralist” and a “threefold realist” (1978, pp. 148–151).


Monists believe that there is only one world: both bacteria and symphonies are part of it. It is clear by now that Popper does not want to equate these two phenomena.


Dualists, on the other hand, would accept the existence of Popper’s World 2 and defend the gap between the brain and the mind. However, for them, Beethoven’s Fifth Symphony does not have an objective ontology. According to dualist, the symphony must be identified with the experiences of those who listen to it and remember it; they reduce World 3 to World 2.


The pluralism or threefold realism of Karl Popper acknowledges the main beliefs of monists and dualists but extends them to the world of human artifacts (World 3). He writes:


“I am a realist regarding the physical world 1 (…) Similarly, I am a realist regarding world 2, the world of experiences. And I am a realist regarding world 3 (…) such as languages; scientific conjectures or theories; and works of art.”
(Popper, 1978, p. 51).


Some Shortcomings of Karl Popper’s Three World Theory

ludwig beethoven
Ludwig van Beethoven, portrait by Karl Stieler, 1820, via Wikimedia Commons.


Dave Elder-Vass, a Critical Realist and sociologist, identifies a problem in Popper’s account. A library contains potential knowledge and not knowledge as such; in other words, books contain only representations of ideas. In the absence of a reader, linguistic marks on a piece of paper are simply not knowledge.


A further difficulty is implied: “(…) different readers in different contexts may interpret the marks in books differently. (…) There is no definitive ideational content contained within a book” (Elder-Vass, 2012, p. 43). The point made by Elder-Vass can be understood by thinking about Egyptian Hieroglyphics. The ancient Greeks had already tried to decipher them. Though scholars have made progress in understanding hieroglyphics, and though many hieroglyphic texts have been translated, there are still many challenges, and additional discoveries are always expected. Furthermore, when decipherment is attempted, the hieroglyphic text needs to be interpreted in the light of contextual knowledge, like historical accounts.


In a nutshell, knowledge (like that embodied in hieroglyphics) is not transparent. Therefore, we can cast doubt on the objective existence of the world 3. Elder-Vass concludes: “Within books, then, there is no knowledge or culture, only marks that may be used to communicate them; and when that communication is completed successfully, what is produced is subjective (World 2) and not objective (World 3)” (2012, p. 43).


egyptian hieroglyphics
Egyptian hieroglyphic inscription on wood, from the tomb of the general Horemheb. Saqqara, Egypt, c. 1300 BCE. In the Louvre Museum, Paris, photo by Jan van der Crabben, via World History.


An additional weakness of Popper’s conception is that it postulates two additional worlds we can do without. According to some philosophers, the best way to explain differences between the brain (World 1), the mind (Word 2), and human artifacts (World 3) is by applying the theory of emergent properties.


Popper disagrees with monists because they conceive a flat ontology in which everything is reducible to physics and chemistry. But there is another type of monism in which emergent properties have a paramount role. The mind is an emergent property of the brain: only when certain components are arranged in a specific way can we say that there is a mind. The mind is not reducible to the brain, but it nevertheless needs it to exist.


mckenzie brain
Untitled by Lucy McKenzie, 2002, via MoMA.


The social ontology of John Searle, for instance, is based on the idea that social artifacts (like money) are the product of emergent properties of human collective arrangements (1996). To use Popper’s terminology, there is nothing about a piece of paper (World 1) that makes it a US dollar. But a dollar is not an object of the World 3 —as Popper would like to believe— because the status of that piece of paper is the product of emergent properties and collective agreement. Along these lines, some have tried to complement Popper’s view with the philosophy of John Searle (Gödert & Lepsky, 2019).


As a philosophical figure, Popper will remain one of the greatest philosophers of science of the past century. To confirm this, one just needs to remember that, for example, he debated with Ludwig Wittgenstein on October 25, 1946. He also became one of the main antagonists of Thomas Kuhn’s philosophy (Fuller, 2003). His ideas will be worth revisiting for a long time to come. Overall, Karl Popper’s Three World theory contributes to a long-lasting discussion surrounding the ontological status of knowledge and culture.




Elder-Vass, D. (2012). The Reality of Social Construction. Cambridge University Press.

Fuller, S. (2003). Kuhn vs. Popper. The struggle for the soul of Science. Icon Books UK.

Gödert, W., & Lepsky, K. (2019). Reception of externalized knowledge: A constructivistic model based on Popper’s Three Worlds and Searle’s Collective Intentionality. http://eprints.rclis.org/34317/

Popper, K. (1978, April 7). Three Worlds. The Tanner Lecture on Human Values, University of Michigan.

Popper, K. (1979). Objective Knowledge. Clarendon Press.

Searle, J. (1996). The construction of Social Reality. Penguin Publishing Group.

Thornton, S. (2022). Karl Popper. In The Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/archives/win2022/entries/popper/

Walker, M. P. (2017). Why we sleep: Unlocking the power of sleep and dreams (First Scribner hardcover edition). Scribner, an imprint of Simon & Schuster, Inc.

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By Andres Felipe BarreroMA Philosophy, MSc Philosophy, Ph.D. CandidateAndrés has a background in philosophy from Universidad de la Salle in Bogotá, Colombia, where he finished his undergraduate and master`s studies. He completed a second master's at Universität Hamburg, Germany, where he wrote about philosophical theories of Modernity and Secularization. Currently, he is a Ph.D. Candidate at Universität Bremen. His fields of interest include the Philosophy of Language, Philosophy of Religion, Philosophy of Science, Social Theory, Discourse Studies, Corpus Linguistics, and Natural Language Processing.