Popper's three worlds

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Popper's three worlds is a way of looking at reality, described by the British philosopher Karl Popper in a lecture given in August 1967.^[1] The concept involves three interacting worlds, called World 1, World 2 and World 3.^[2]

Worlds 1, 2 and 3[]

These three "Worlds" are not proposed as isolated universes but rather are realms or levels within the known universe.

Their numbering reflects Popper's view (a) of their temporal order within the known universe, and (b) that each later realm emerged from developments within the preceding realm. A one-word description of each realm is that World 1 is the physical realm, World 2 is the mental realm, and World 3 is the cultural realm - though, in the detail of Popper's theory, each "World" or realm transcends what might be typically understood by the respective terms "physical", "mental" and "cultural".

While each major realm (i.e. World 1, 2 and 3) is said by Popper to arise from the emergence of new entities, Popper accepts that multiple levels of new entity may also be distinguished as having emerged within each realm e.g. physics, chemistry and biology are subjects corresponding to distinguishable levels within Popper's "World 1", and it is possible to distinguish further levels within these levels (e.g. within chemistry, between inorganic and organic chemistry). In Popper's view, many of these levels emerged so gradually that it may not be feasible to precisely demarcate their borders, and this may be the case regarding the emergence of World 2 from World 1 and of World 3 from World 2. Nevertheless, Popper's tripartite division of all distinguishable emergent levels in the evolving universe, into three major distinct realms or "Worlds", reflects his view that the various entities within each respective "World" share crucially distinct characteristics when compared to entities in the other "Worlds", and that the points of interaction between these three "Worlds" are of special interest and importance.

In Popper's theory, there is at points direct interaction between World 1 and World 2 (at the point of mind-brain liaison), and between World 2 and World 3 (at the point where mental states are engaged with World 3 content), but there is no direct interaction at any point between World 1 and World 3 - World 3 does affect World 1 but only indirectly, through the workings of World 2 with World 3 content and then World 2's subsequent interaction with World 1 via mind-brain-body interaction.

Though it is part of Popper's theory of knowledge, Popper's theory of these three "Worlds" is also a cosmological theory. Consistent with the known universe as presently described by the natural sciences, Popper maintains that the known universe did not contain any World 2 or World 3 from the outset - there was initially only a "World 1", a realm where everything consisted of physical states and processes, with chemical reactions between the elements arising much later. Moreover, that "World 1" was for an even longer time devoid of any living matter, and so during that time lacked any biological level. The biological level is a level within World 1 that emerged from its physical-chemical evolution over a vast tract of time, as a lifeless universe eventually gave rise to living organisms. In a similar sense to this emergence of life within World 1 itself, Popper maintains that a "World 2" of mental life later emerged as a product of biological evolution, and that subsequently "World 3" emerged as a product of evolution within the human "World 2". Much as there was no biological realm in the universe before the emergence of biological entities, so Popper views "World 2" and "World 3" as realms that only came into existence with the first respective World 2 and World 3 entities.

This cosmological approach is directly opposed to any form of reductionism which claims that we may eventually be able to fully explain (and perhaps predict) whatever comes later in the known universe from the character of what came before - or claims that, in principle, we can always completely 'reduce' later characteristics of the universe to its earlier characteristics. Against such reductionist and deterministic views, Popper argues that we should view our universe as "creative" and indeterministic, especially in that it has given rise to genuinely new levels or realms - like biological life, "World 2" and "World 3" - that were not there from the beginning and which are not 'reducible' to what was there from the beginning.

The three "Worlds" may be understood, within this evolutionary and cosmological framework, as containing three categories of entity:-

• World 1: the realm of states and processes as typically 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. Mental states and processes only emerged as a product (or by-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-motivated human action on World 1 - have an indirect but powerful affect 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.

Popper recognises that some form of World 1 and of World 2 is accepted by most people, but that his "World 3" goes beyond what is generally accepted as a matter of common sense. While humans may be widely considered to have developed many forms of "culture", it may often be assumed that these forms can be entirely explained in terms of World 1 and/or World 2 e.g. in terms of physical artefacts, or dispositions to act or memories physically encoded in the brain; or in terms of subjective knowledge or psychological dispositions within World 2. Whereas, in Popper's theory, those physical and mental entities which embody or represent cultural forms should be understood as vehicles for the relevant World 3 content e.g. the cultural content of a painting or musical composition or a book does not reside in their characteristics in World 1 terms (i.e. their physical, chemical or biological aspects as studied by the natural sciences), or in their World 2 affects on the human mind (as perhaps studied by psychology), but in their distinctive World 3 content - which, Popper argues, is not identical with, or reducible to, any World 1 or World 2 state or process.

In addition to (1) World 3 content that is embodied or encoded physically in World 1 as artefacts like books or as brain contents such as stored memories ("World 3.1" content), and (2) World 3 content that has been the past or present subject matter of mental states and processes ("World 3.2" content), it is an important part of Popper's theory that there is also (3) "World 3.3" content i.e. there also exists, Popper argues, World 3 content that has as yet had no existence in the form of a World 1 or World 2 'vehicle' e.g. prime numbers existed as discoverable "World 3.3" content, as an immediate 'built-in' consequence of the sequence of the natural numbers, as soon as the sequence of natural numbers was developed by humans - and therefore prime numbers existed in "World 3.3" before their existence was either grasped mentally by humans or given any specific physical embodiment in mathematical speech or writing. In Popper's view, humans invented rather than discovered the sequence of natural numbers, but humans discovered rather than invented the existence of prime numbers within that sequence - and he suggests his theory may throw light on other debates as to whether some particular item of knowledge has been discovered or invented. Popper implicitly recognises "World 3.3" content with his "theorem" that "World 3 > World 3.2 + World 3.1" - meaning that World 3 transcends those World 3 contents that are either encoded in World 1 or grasped by World 2. This theorem also means that, in Popper's view, World 3 contains not just contents that are the intended products of human mental activity but also contents that are unintended by-products of other World 3 contents. For example, Mendel's work, showing genetic material is 'particulate' (and combines but does not 'blend' in reproduction), existed in World 3 together with Darwin's theory of 'natural selection' before anyone understood how their theories interconnect; so while Mendel and Darwin invented their theories, important connections between these theories existed to be discovered in "World 3.3" once the theories themselves existed in World 3 e.g. Mendel's theory explains why advantageous mutations are not blended out by subsequent interbreeding before they can further spread in the gene pool, thereby answering one of the most critical objections to Darwin's theory, an objection that Darwin himself struggled to answer because he was unaware of Mendel's work.

As already indicated, it is crucial to Popper's theory that where World 3 content is embodied physically or grasped mentally it nevertheless retains its character as World 3 content (hence the aptness of the terminology of "World 3.1", "World 3.1" and "World 3.3" content, which marks the distinctness of World 3 content even where it has a World 1 or World 2 vehicle) e.g. in Popper's view, the World 3 content of a book remains within World 3 and does not exist within World 1 as content of the sort studied by physics, chemistry or biology (accordingly, two editions of a book may be identical in World 3 terms despite marked differences in their physical and chemical composition). That is, Popper's theory is that World 1 objects can provide vehicles for World 3 content, but such World 1 vehicles do not fully embody the World 3 content because, crucially, the World 3 content retains its World 3 character rather than being converted into some form of World 1 content. This may at first seem a strange view, but Popper argues that the act of reading is an interpretative act of the conscious mind seeking to grasp World 3 content, and is clearly not an act of merely 'reading' physical or chemical characteristics in World 1 terms - with the World 1 aspects of writing important only in their role as symbolic vehicles for the relevant World 3 content, and otherwise unimportant in any specific World 1 terms (they could always, in principle, be replaced by a physically different symbolism). That World 3 content may remain invariant between translations into different languages that differ markedly in World 1 terms; and where there are difficulties of translation (i.e. in keeping the content invariant in translation) these are not World 1 difficulties but World 3 difficulties e.g. they are difficulties in finding sufficiently equivalent expressions in the other language to convey relevant aspects of the World 3 meaning. The initial strangeness of this "World 3", in contrast to the more familiar concepts of World 1 and World 2, may also be counterbalanced by understanding that World 1 and World 2 are shown by science to be much stranger and more complex than they might appear to "common sense", and even to defy "common sense" e.g. quantum physics within World 1; perceptual and other cognitive illusions within World 2.

World 1 extends beyond what is testable by science to those aspects of physics, chemistry and biology that remain a matter of philosophical or metaphysical (i.e. untestable) conjecture - so, for example, arguments as to whether there exist any 'laws of physics' pertain to World 1 even though neither the existence nor non-existence of such laws can be tested (i.e. falsified) by observation (only forms of specified law can be falsified or tested, in Popper's view, not the existence or non-existence of laws in general terms). Also pertaining to World 1 are untestable theories as to the origin of World 1 and the origin of life within it. Similarly, a proposition may pertain to World 2 or World 3 whether it is scientifically testable or not. Deeper than this, Popper does not claim our knowledge (whether scientific-testable or not) is, or ever could be, exhaustive of the reality it seeks to comprehend. His position is that even were we to reach apparent 'rock bottom' in our understanding of, say, the laws governing the physical universe, our knowledge would always fall short of conclusive proof that we had reached the actual 'rock bottom'. Moreover, it may be that World 1 has aspects to its reality, including aspects fundamental to its physical nature, that we shall never understand - and perhaps that we are incapable of ever understanding. Equivalent points also hold true for World 2 and World 3.

To appreciate Popper's theory it helps to appreciate that:-

(a) fundamental aspects of World 1 and World 2 defy "common sense" and might be regarded as extravagant and fantastical were they not shown by critical arguments, including experiments;

(b) the reality of World 1 and World 2 are not themselves straightforwardly testable in the sense in which their reality is usually disputed by philosophers, and their reality is better understood as a conjecture made as part of our metaphysical outlook;

(c) we have not got to the bottom of understanding either World 1 or World 2, and the deeper our understanding has penetrated into these realms the more we have unearthed aspects that are not obvious as a matter of common sense or ordinary experience;

(d) we do not have any general theory of causation that explains the causal interactions within each realm and between each realm;

(e) there are key aspects of human knowledge, including language acquisition and extending to scientific knowledge, that are difficult to fully explain in terms of subjective mental states and which are much better explained by a theory of World 3 and of "objective knowledge".

Viewed in such a wider perspective, the reality of "World 3" may seem less extravagant and fantastical, and less vulnerable to objections that it is not part of common sense and is not demonstrated by a clear or testable explanation of how causally it may interact with World 2. In Popper's view, the reality of World 3 is a reasonable further metaphysical conjecture in line with the conjectured reality of World 1 and World 2:- it is made more reasonable by our increased knowledge of the evolving character of World 1 and World 2, especially that discovered aspects of their reality extend far beyond the visible or graspable material objects revealed in ordinary experience (and which arguably form the core of ordinary experience) - so that the 'abstractness' of World 3 should be viewed relative to the 'abstractness' (by contrast to the experienced 'concreteness' of visible or graspable material objects) of atoms, sub-atomic particles and forces, and of 'natural selection' and 'relativity' within World 1, and of mental states and processes within World 2 - all of which we have reason to consider as "real".

Among the many points and arguments Popper makes using his three worlds' schema, the following may be regarded as key claims regarding the role of World 3 in the known universe.

First, Popper argues that, notwithstanding 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 knowledge of 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 a nuclear reactor is not the result of a purely World 1 process but is the product of a complex interaction between particular "World 3" theories and human "World 2" mental activity, and then particular human actions in "World 1" made possible by this complex interaction.

Third, Popper argues that one way to bring out the reality of World 3 is to consider how its existence drastically changes the state of World 1. He gives the contrasting examples of a disaster that wiped out civilisation including our stored knowledge (e.g. in libraries and in computers) and a similar disaster which left our stored knowledge substantially intact. In the first example, civilisation as we now know it, including its knowledge-base, would have to be rebuilt from scratch; whereas, in the second case, even if all the relevant experts had died in the disaster, we could avail of the stored knowledge-base to begin rebuilding. Popper argues that the stored knowledge-base is World 3 content in its physically encoded form - "World 3.1" - and its reality becomes clear when we consider how much more difficult the rebuilding of civilisation would likely be in the first example when contrasted with the second, with the 'rebuilding of civilisation' being a process that would have many World 1 effects.

Contrast between Popper's theory of knowledge and the theory of knowledge as "justified true belief"

In contrast to the dominant Western tradition that identifies "knowledge" with special states within World 2 (e.g. with "justified true belief"), in Popper's theory of knowledge there exist forms of knowledge within World 1 as well as within World 2, and additionally there are knowable 'objects' within World 3 which constitute forms of "objective knowledge". Objects or entities within each of these "Worlds" constitute the subject matter of human knowledge. Popper's interactionist view is that human knowledge arises from combinations of knowledge in World 1 terms (e.g. processes built into the brain), "subjective knowledge" in World 2 terms, and "objective knowledge" in World 3 terms.

Central to Popper's overall theory of knowledge is his contention that a form of "making and matching" or "conjecture and refutation" is the model by which knowledge develops at all these levels i.e. within World 1, and within World 2, and also as regards knowledge arising from World 2's interactions with World 1 and World 3.

Popper's theory of knowledge is very different to the view that identifies knowledge with some form of "justified true belief", and he argues, by contrast, that:-

(a) all knowledge is conjectural, because it always might be mistaken, and none is "justified" in the sense philosophers usually mean i.e. of having an infallible basis;

(b) knowledge encompasses what is false as well as what is true e.g. Newton's physics, though strictly false, is highly significant knowledge - much more significant than knowledge that is true but trivial;

(c) (i) knowledge need not be believed or involve 'belief states' (e.g. knowledge stored in libraries; discoverable knowledge in the "World 3.3" sense - that has not, as yet, been the subject matter of any human belief);

(ii) we may be more committed to our knowledge the stronger our convictions or 'belief-states', and this commitment or conviction may sometimes appear useful as a well-spring for action, but commitment and conviction may also underpin dogmatism and impede critical correction of our knowledge, and we should pursue knowledge wary of such commitments and convictions;

(iii) the value of knowledge stands in no strict correlation with our subjective degree of belief in it;

(iv) obviously the "value" of knowledge may depend on the values we adopt (and some knowledge may be invaluable because it is essential to survival, such as knowing how to breathe without conscious effort), but, if we take truth as a core value and consider knowledge for its explanatory power, then the value of knowledge centres on (a) how well it stands up to critical examination in terms of its truth or 'truth-likeness' and (b) its degree of explanatory power. The truth or 'truth-likeness' of knowledge and its explanatory power are issues we strive to answer in objective terms, by critical evaluation of the relevant "objective knowledge", and they cannot be properly answered by any mere appeal to subjective mental states, including our degree of belief.

According to Popper's views on knowledge:-

(1) a tree may have knowledge, such as how to spread its roots to tap water; but this is biological knowledge within World 1 - a tree does not have mental states or World 2 knowledge;

(2) in addition to such biological knowledge, explicable in World 1 terms, a dog may have mental states and World 2 knowledge - such as a conscious expectation as to when its owner is likely to return home (in Popper's view, the dog's mental state will not involve knowledge of 'owner' or 'return' or 'home' in their World 3 sense, but still there may be some resemblance between the dog's understanding of the relevant situation in its World 2 terms and human understanding of the same situation in World 3 terms - so that, for example, the dog attaches special significance to specific persons, actions and places in a way that resembles what humans would signify and explain in terms of 'owner', 'return' and 'home');

(3) only humans have knowledge that extends beyond World 1 and World 2 states and processes to knowledge of 'objects' or entities in the World 3 sense.

More on world 3[]

Popper's world 3 contains the products of thought. This includes abstract objects such as scientific theories, stories, myths and works of art.^[3] A world 3 object is something along the lines of a meta-object or a form of being. 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.^[4] It corresponds to the current state of our knowledge and culture.^[5]

The interaction of World 1 and World 2[]

Popper's theory of the interaction of World 2 and World 3 may be said to provide a decisive step beyond mere mind-body dualism into a form of 'trialism', yet within that 'trialism' there is a distinctive theory of mind-body dualism - one that provides an alternative to Cartesian dualism.

Notwithstanding Popper's World 3 'trialism', it is clear that Popper accepts that there was a time, before World 3 emerged, when there were only World 1 and World 2 entities - and therefore there was at that time only a mind-body dualism. Popper regards Descartes as correct in his recognition that we must take mind (or 'the mental') as being something crucially distinct from body (or 'the physical'), and in that sense Popper's position remains in the tradition of Descartes. But Popper considers that Descartes made two fundamental errors - first, in taking these distinct entities of mind and body to be constituted as distinct "substances", and, second, in applying a restrictive theory of causation only by 'push' to both the physical world and to the interaction of that physical world with 'mind'. In contrast to Descartes, Popper also allows that animals other that humans may have mental states and processes (e.g. dogs can be shown experimentally to have dream sleep); and Popper also allows for the existence of unconscious mental states and processes, and for differing degrees of conscious awareness in relation to mental states (e.g. an individual human may experience different degrees of conscious awareness in relation to their mental states in the course of the day, depending on factors like tiredness or whether they are in an emergency or are sleeping). Consequently, Popper's position is distinguishable from Cartesian dualism and provides a form of dualist interactionism that is not vulnerable to all the same criticisms.

Popper describes himself as a dualist in terms of mind and body, and he rejects any form of 'identity theory' whereby mental states are taken to be the same as physical brain states. Nevertheless his approach allows (1) for mental states to be in some sense dependent for their existence on a physical substrata in the brain; (2) for there to be close interconnectivity between mental states and physical brain states; (3) for many, if not most, mental states to be products of World 1 brain states and processes without in turn having any affect on any brain states or processes. Given this interconnectivity and interdependence between World 2 and the World 1 of brain activity, it is clear that when Popper speaks of the "autonomy" of World 2 from World 1 he does not mean complete autonomy in every respect - he primarily means "autonomy" in certain specific instances of mental processing, so that the World 2 of the human mind may be regarded overall as having partial autonomy from its World 1 brain. In Popper's theory, this partial autonomy is then amplified by World 3:- because World 2 thought processes, when they are autonomous from World 1, can create autonomous World 3 objects - and World 2's further interaction with those World 3 objects gives World 2 greater scope for the further pursuit of knowledge unbridled by the limitations of World 1 processes.

For Popper, dualism alone is not enough to grant the human mind any creative freedom or autonomy from its World 1 brain - for example, a mind-brain parallelism could be dualistic even if we lacked genuine mental freedom or rationality because our mental states were always mere epiphenomena to World 1 brain activity and so had no causal effect on World 1. It is in defence of human creativity, rationality and freedom that Popper stresses the 'causal openness' of World 1 to World 2 (and of World 2 to World 3) - that is, Popper's position is 'interactionist', not merely 'dualist'. Accordingly, it is a crucial part of Popper's view of World 1 and World 2 interaction that mental states can sometimes causally affect physical brain states i.e. mental states or processes can produce changes in World 1 brain states. For example, a human's deliberate conscious and rational action involves all of the following:- (a) a mental state (or process) that has some form of autonomy from World 1 brain states and processes, including those that may have helped produce it; (b) that mental state (or process) effecting changes to World 1 brain states; (c) the changed World 1 brain states producing World 1 action by the body. Popper argues that critical discussion favours the conclusion that there occurs such 'downward causation', i.e. of mental states/processes changing physical brain states/processes, even though we do not know how it occurs.

Popper argues that some form of mind-body dualist interaction has been the predominant position throughout human history, both philosophically and as a matter of common sense, and as reflected in the law. This dualism was taken as relatively unproblematic until the time of Descartes when it became a pressing concern to explain how something as possibly ethereal and scientifically untestable as 'mind' could fit within the materialistic and mechanistic conceptions of the Scientific Revolution. Popper proposes that mind-body dualism subsequently suffered because it became identified with "Cartesian dualism", which turns out to be an untenable form of dualist interactionism. In Descartes' approach, mind and body must interact by one 'pushing' on the other, as 'push' is the only form of causal connection accepted by Descartes. Yet in Descartes' view both mind and body are substances, the difference between them being that bodies are material substances and the mind consists of an immaterial substance. This proves untenable as a scheme of explanation because it is impossible to see how an immaterial substance (here 'mind') could 'push' on material bodies (say, the brain) when 'push' is conceived in a purely material way.

In opposition to this scheme of explanation, Popper rejects both the view that minds and bodies should be regarded as "substances" and Descartes' view that all causation is some form of 'push'. Popper also rejects those post-Cartesian theories that might appear to circumvent the causal issues, thrown up by interactionism, by denying there is any mind-body interaction - whether the denial takes the form of 'identity theories' that assert mental events are just physical brain events, or takes the form of a mind-body parallelism, such as epiphenomenalism, which denies mental activity has any effect on the physical world. While materialistic identity theories might seem very different to dualistic theories like epiphenomenalism, Popper argues that both are in the same boat in terms of denying that mental states have any causal effect on World 1, and that this brings both into conflict with a Darwinian explanation for the evolution of mental states which must be based on their causal effects in terms of World 1 adaptiveness, reproduction and survival. More broadly, Popper also rejects a narrowly materialistic view of the universe, arguing that, since the Scientific Revolution, the programme of explaining everything in terms of matter has been transcended within science itself e.g. by the need to explain matter in terms of non-material forces. Popper's position is readily allied with the view that many intuitions and arguments (a) against the reality of an autonomous, causally affective World 2, and (b) for regarding World 1 as causally closed (i.e. as 'cut off' from World 2 influence), are based on an outdated and overly simplistic view of the character of World 1.

Popper admits that we do not know how such different entities as mind and bodily brain could causally interact, but he argues this should not be regarded as an insuperable difficulty for dualist interactionism - for we do not know how it is possible for an invisible disembodied force like gravity to causally affect huge amounts of embodied matter like stars, but rightly do not regard this as an insuperable objection to a theory of gravitation. In short, Popper argues, we may know (or 'best guess') that certain different entities interact - even though in some fundamental sense we neither know how such interaction is possible nor know precisely how it may happen.

Popper consistently rejects purely 'physicalist' thinking in the area of understanding mind-body liaison. For Popper, we should avoid prescribing that only a physical model of causality can apply when understanding mind-brain interaction - for we may know that a stock market crash will likely cause a rise in unemployment, or that a conductor dropping a baton may cause the orchestra to falter, while knowing that the model of this causality cannot be a matter of pure physics. Popper also argues that we may rationally place more weight on how we actually experience mental events 'from the inside', as evidence that we actually have distinctly 'non-physical' mental experiences, than on the 'physicalist' argument that distinctly non-physical mental events do not exist because these are not physically detectible in the way physical events are. In Popper's view, we may even regard subjective experience as scientifically testable, and as passing certain tests, provided we allow our mental experience to provide evidence for the test e.g. we can test for the existence of non-physical mental experience by gently pressing on our eyelid and then experiencing both the fracturing of the visual image and conscious awareness that this is due to pressure on the eyelid and not to the fracturing of the external physical world - obviously this test is not conclusive to disprove certain philosophical doubts, but it is not merely question-begging as it uses the specific character of the experience to argue for its non-physical existence, and that specific character is also inter-subjectively testable as any human may undertake the test for themselves.

In Popper's version of dualist interactionism, human mental states and processes arise from extremely complex and intensely active World 1 processes in the brain. Such is the level of complexity and activity that it may be somewhat misleading to speak of a mental state or process in singular terms, as the actual workings of mind and brain may arise from interacting complexes of multiple physical and mental states and processes - in this regard, referring to a mental or brain state or process as if they are a singular entity may be understood as just a useful simplification and not a reflection of the more intricate reality where the state or process is a complex composite. The World 1 activities of the brain may be analysed in terms of physics and chemistry but also biology, as the human brain is a living and highly active organ - the most complex and most intensely active entity within World 1 - with this giving rise to consideration of the biological purposes of brain activity. Popper allows that in many cases mental events may be mere causal upshots of World 1 brain processes, running parallel to the brain processes without affecting those processes in turn. But, considered in terms of biological purposes, Popper argues that it does not fit with an evolutionary view of the mind to consider mental events as never producing any effect on the bodily brain:- for the evolution of greater mental capacity (that we may trace in the 'higher' animals compared to the 'lower') would have no rationale in terms of 'natural selection' if that greater mental capacity had no causal impact on the animal's actions and behaviour. Such evolution is very hard to rationally explain without recourse to 'natural selection' and it is very hard to apply 'natural selection' to the evolution of entities that play no causal role in survival and adaptive behaviour. Therefore there must exist instances of "downward causation", Popper argues, where some mental states or processes somehow sometimes act 'downwards' to alter brain states and thereby action and behaviour, if the evolution of human consciousness is to be understood as conferring any adaptive advantage in terms of 'natural selection'.

Popper's theory of World 3 takes this last argument a step further, by regarding the mind's development of World 3 as also conferring adaptive advantages, and with World 3's continued evolution being at least partly explained by the advantages conferred. In Popper's theory, the human mind's ability, to create and interact with autonomous World 3 'objects', massively amplifies its ability to develop knowledge that is autonomous from World 1 brain states and processes.

Though the World 2 evaluation of World 3 objects is cultural rather than 'natural selection', there are, in Popper's view, many analogies to be drawn between cultural and natural evolution - especially that both work along the lines of non-inductive/Darwinian 'selection between variants' and not by inductive/Lamarckist 'instruction from environment'. The central adaptive advantage of human cultural evolution is that we can let our theories die in our stead, by having our knowledge advanced through non-violent and non-lethal intellectual competition. Human knowledge has advanced in depth and breadth beyond the knowledge attained by other animals, and this, in Popper's view, is bound up with the development of World 3 and the expansion of humans' World 2 interaction with World 3 contents. According to Popper's theory, the depth of our cognition and the scope of our error elimination exceeds that of other animals because we are able to articulate our knowledge as World 3 objects (e.g. theories) that have greater range and depth than what is otherwise presented by sensory or subjective experience in World 2, because we are able to investigate these World 3 constructs as "objective knowledge", and because we are able to subject them to much more rigorous and systematic testing and assessment than is otherwise possible with knowledge that merely exists in the form of subjective mental states.

So Popper's theory posits two cases of interaction - World 3 with World 2 and World 2 with World 1 - and Popper maintains that both cases of interaction may be rationally defended even though in both cases we lack a theory of causation that adequately explains how this interaction occurs. Popper argues that, in truth, we lack such a theory of causation in general terms - even for just the different kinds of interaction within World 1 (e.g. the different physical forces with different particles; the effect of structures on their components and vice versa). Scientific advances have shown our primitive views of World 1 causation (e.g. as materialistic 'push') to be highly inadequate (what we might mistake as simple push between two objects, like the hammer on the nail, is in fact a complex interactive process mediated by force fields). Yet the myriad complexity of interactions within World 1, as revealed by science, is not underpinned by any general theory of causation or explanation of how those interactions are causally possible:- many fundamental interactions, such as forces with matter, are asserted as part of scientific theories without science further explaining how these interactions are fundamentally possible. That is, scientific theories generally present no testable explanation of how their theorised 'interactions' are causally possible, in the sense under discussion:- e.g. a theory of the interaction of gravity and mass may posit a two-way relationship where gravity affects the behaviour of material mass and material mass affects the operation of gravity, without any testable explanation of how this two-way relationship is causally possible. In this light, Popper argues, we should not regard the shortcomings in our understanding of how a form of interaction is causally possible as anything like a decisive argument against the existence of such interaction.

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 into an object of 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.^[6][7]

See also[]

• Eccles' philosophy
• Platonic realm
• Third Realm (Frege)
• Trichotomy (philosophy)
• Group mind
• Logical Investigations

Notes and abbreviated references[]

References[]

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 (1994). Knowledge and the Body-Mind Problem.
• Popper, Karl (1994) [First edition 1945]. The Open Society and Its Enemies. Routledge 2012, Princeton University Press 2013.{{cite book}}: CS1 maint: date and year (link)