Popper's three worlds

Popper's three worlds is a way of looking at reality, described by the Austrian-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 material 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 "material", "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; within biology, the different levels of entity or species identified by evolutionary classification). 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 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.

Popper uses the framework provided by the interactions of the three "Worlds" to advance his account of human creativity, rationality and freedom. Popper's position is that there is continuity and similarity between many aspects of human knowledge and the knowledge attained by other animals and living entities, and he argues for an "evolutionary epistemology" which regards all such knowledge as instruments for problem-solving - instruments that evolve, using "conjecture and refutation", according to a schema of 'initial problem - trial solution - error elimination - revised problem'. Yet he argues that what most separates human from animal knowledge is that only humans develop their knowledge in World 3 terms:- he contends that much of the vastly enlarged human capacity to be creative and/or rational, in comparison with the capacities of other animals, arises from humans' unique access to "objective knowledge" in World 3 terms i.e. from the creative, rational and indeterministic interplay between human "subjective knowledge" in World 2 terms and "objective knowledge" in World 3 terms - the interplay between the human 'mind' and the many forms of human 'culture'. Within this framework, Popper defends the view that humans are moral beings with a crucial degree of rational 'autonomy', but he argues we are not born fully fledged 'selves' or with rational autonomy but that we achieve selfhood and develop rational autonomy as our minds develop in interaction with other selves and with World 3.

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 or processes 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, for an even longer time that "World 1" was 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. Likewise, when Popper speaks of the interaction between, say, "World 2" and "World 3", he means interaction between specific entities that exist within these respective realms rather than interaction of the "Worlds" as such.

This cosmological approach is directly opposed to claims that we will 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 its beginning, and which are not 'reducible' to (or predictable from) what was there from its 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 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.

World 3 content and its vehicles in World 1 and World 2

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 a 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. This World 3 content, Popper argues, is not identical with, or reducible to, any World 1 or World 2 states or processes.

In addition to:-

(1) World 3 content that has been 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' and which only exists within World 3 itself.

Popper often uses the example of prime numbers and their relation to the sequence of natural numbers to illustrate his form of objectivism. Though his objectivism extends to other World 3 entities, such as works of art and scientific theories, it would appear that Popper considered mathematical examples as a good way to convey his point of view. In Popper's view, primes existed as discoverable "World 3.3" content in the above sense, as an immediate 'objective' property of the sequence of the natural numbers, as soon as the sequence of natural numbers was developed by humans - accordingly, prime numbers existed in World 3 before their existence was grasped mentally by humans or was made explicit in mathematical speech or writing. In Popper's account, humans invented rather than discovered the sequence of natural numbers (a sequence which does not exist within World 1 and which did not come into existence with the first human mental states), but humans discovered rather than invented the existence of prime numbers within that sequence; so, in terms of the three "Worlds", natural numbers came into existence with their conscious invention and use by humans ("World 3.2" content), but prime numbers entered World 3 at the same time as an unintended but discoverable "World 3" property of the sequence of natural numbers ("World 3.3" content). Popper suggests this approach may help 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 have been 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, as "World 3.3" content, once the theories themselves existed in World 3 (e.g. Mendel's theory can be used to explain 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).

Even in the case of the World 3 objects that are the intended products of human mental activity, Popper contends that such objects may have a depth of content that transcends the human understanding involved in their creation e.g. Einstein's "e = mc²", Darwin's theory of 'natural selection', Shakespeare's "King Lear", Joyce's "Ulysses", Bach's "Well-Tempered Clavier", Dylan's "Highway 61 Revisited", or Leonardo's 'Mona Lisa', may have depths that were not fully known or understood by anyone at the point of their creation. Indeed, Popper's theory is that some World 3 objects have sufficient depths that their content can be 'mined' for an indefinite time after the object was initially created, with some of that 'mining' of considerable importance in terms of advancing our understanding e.g. 'mining' the sequence of natural numbers for properties and relations such as primes, and other different sets and classes of number and number relation; R.A. Fisher's explanation for the prevalence of 1:1 male-female ratios in terms of Darwin's 'natural selection'; a particularly insightful performance of a play or musical composition; an especially penetrating commentary on a book or work of art.

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 3.2" 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; and a single edition of a book may remain unchanged in World 3 terms despite changes in its physical and chemical composition as it deteriorates over time). 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, for example, 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 or chemically 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 mostly 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 intuitively 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.

The reality of World 3 in the context of the reality of World 1 and 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 specified 'laws of nature' can be falsified or tested, in Popper's view, whereas there is no observational test for the existence or non-existence of such 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, and untestable theories regarding the extent to which World 1 is governed by exceptionless laws of nature, or governed by probabilistic laws, or by probabilistic 'changing propensities for change', or is a product of random or blind chance. 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, especially the reality of a "World 3", it helps to appreciate that:-

(a) fundamental aspects of World 1 and World 2 defy naive forms of "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 the view that they are "real" is better understood as a conjecture made as part of our realist 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 everyday experience;

(d) we do not have any general theory of causation that explains (i) the causal interactions within each realm and/or (ii) 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 untutored 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 touchable material objects central to ordinary experience - so that the 'abstractness' of World 3 objects (by contrast to the experienced 'concreteness' of visible or graspable material objects) should be viewed relative to the 'abstractness' of atoms, sub-atomic particles, gravity and other forces, the spectrum of electro-magnetic waves, 'natural selection', 'relativity' and 'space-time' within World 1, and of conscious and unconscious mental states and processes within World 2. Popper argues that a main reason we may consider all these entities as "real" is because their existence is needed to explain the character or behaviour of the visible or touchable material objects which arguably form the core of ordinary experience.

Key aspects of the role of World 3

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" content - 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. The "World 3.1" stored knowledge-base (e.g. in libraries and in computers) is itself an array of World 1 objects whose character and behaviour, Popper argues, need to be explained in terms of World 3 content - not just the content which these objects may record or encode but also the World 3 content needed for their creation and maintenance.

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 main subject matter of human knowledge. Popper's interactionist view is that human knowledge often arises from combinations of knowledge in World 1 terms (e.g. information 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) the traditional philosophical search for infallible foundations for knowledge is a fool's errand, because knowledge does not grow or develop by attaining, or seeking, perfect or firm foundations but through improvements to its accuracy and scope; at the human, cultural - and especially scientific - level, these improvements arise from a fallible process of "conjecture and refutation" and from rational criticism, without the knowledge having any conclusively proved infallible foundations;

(d) (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 or because it contributes to our sense of well-being, 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) while we may often be more willing to rely and act on knowledge the greater our degree of belief in it, there is no simple logical connection between our degree of belief in knowledge and our actions performed in reliance on that knowledge e.g. we are capable of deciding to act on knowledge or information in which we have little belief or even which we believe false, and we are capable of refusing to act on knowledge or information which we strongly believe is true; and there are differences between individuals and contexts as regards what standard of assessed reliability is required of "knowledge" before it is relied on for action;

(v) obviously there is a sense in which the assessed "value" of knowledge may depend on the values we adopt (and some knowledge may be invaluable though we do not consciously assess it as such e.g. 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 amount of explanatory power. The truth or 'truth-likeness' of knowledge and its amount of 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, or how to exchange information about the state of its environment with other trees; 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', 'return', 'home' or 'time' in their World 3 senses, 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', 'home' and a specified 'time');

(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.

Differences between Popper's dualism and Cartesian dualism

Notwithstanding his 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. This dualism is in the tradition of Descartes to the extent that it regards the mind (or 'the mental') as being something crucially distinct from the body (or 'the physical'), with a corresponding categorical distinction drawn between World 1 and World 2. But in many other respects Popper's views differ from Descartes', for example on what Popper considers two fundamental Cartesian 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' - both to the physical world and to the interaction of that physical world with 'mind'. In further contrast to Descartes, Popper allows that animals other that humans may have mental states and processes (e.g. dogs can be shown experimentally to have dream sleep); and he 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, providing a form of dualist interactionism that is not vulnerable to all the same criticisms - one which accepts that mental states and processes are highly dependent on physical states and processes in the brain but which emphasises the evolutionary advantages conferred by autonomous World 2 activity in terms of furthering World 1 adaptiveness.

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 (and processes) 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/processes and physical brain states/processes; (3) for many, if not most, mental states/processes to be products of World 1 brain states/processes without in turn having any affect on any brain states/processes. Given this interconnectivity and interdependence between World 2 and the World 1 of brain activity, and especially that the 'causal arrow' between World 1 and World 2 events may often be one-way, 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.

Dualism, like 'indeterminism', "is not enough"

Dualism by itself, Popper argues, 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, his 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 (or processes). 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/processes; (c) the changed World 1 brain states/processes producing World 1 action by the body. In Popper's view, despite the many difficulties and imponderables raised by mind-brain liaison, critical discussion favours the common sense conclusion that there occurs such "downward causation" (i.e. of mental states/processes changing underlying physical brain states/processes), even though we do not know how it occurs.

Problems with Cartesian dualism and mind-brain causation led to mistaken rejection of dualism

Challenging the view that dualism is a Cartesian bogey, 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 as a purely material form of contact.

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. His 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. In particular, Popper argues that mistakenly restrictive views regarding issues of causality prompted many thinkers to wrongly abandon dualist interactionism e.g. many thinkers subscribed to the view that there were two distinct "simple theories of causation, one for World 1 and one for World 2" - material push within World 1, association of ideas and sensory 'data' within World 2 - and the seeming incompatibility of these causal theories made mind-brain interaction appear incomprehensible.

Issues of causality are often regarded as central to the problems with dualist interactionism, but Popper views these unsolved and perhaps unsolvable problems as far from decisive. For example, he admits that we do not know how such different entities as mind and bodily brain could causally interact but 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 often correctly know or conjecture 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's rejection of 'physicalism' and 'reductive materialism'

Not only are World 2 and World 3 proposed as non-physical realms, but Popper rejects purely 'physicalist' thinking in the area of understanding mind-body liaison e.g. 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 their existence is not physically detectible in the same way as physical events. 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 with adequate sight may undertake the test for themselves.

The philosophy of 'physicalism' derives most of its attraction, Popper argues, from the explanatory success of physics and from a programme of 'reductionism' within science that holds out the promise that future developments will eventually see all problems and issues as resolvable in terms of physics (or perhaps physics and chemistry). Despite his highest regard for the explanatory successes of physics as a science, Popper regards 'physicalism' as a mistaken attempt to elevate this success into a monistic philosophy of everything - by contrast, he argues, our increased understanding of the evolution of the universe (itself indebted to physics) shows that there has emerged a plurality of many distinct kinds of entity, from the platypus to the experience of grief to a Beethoven symphony, and that many of these entities have a character that cannot readily be accounted for in terms of physics or chemistry. He emphasises three main points that undermine 'reductionism' as a general philosophy:- (1) attempts at reductions within science have only been partial, if striking, successes - so that, for example, at present science is not even close to reducing the explanations needed for chemistry to those of pure physics; (2) a complete 'reductionism' of everything to physics would have to include reducing scientific explanation itself to a product of physics, and there are many reasons this is not feasible - including that scientific explanation arises from World 2 activity and World 3 theories that cannot be reduced to pure physics; (3) reductionism as a programme within science is rightly pursued because a successful reduction increases testability/falsifiability by having a more general unifying theory supplant two or more disparate theories, but reductionism as a philosophical diktat has the opposite tendency as it involves denying without test the existence of a class of entities (e.g. World 2 mental events) which, if allowed to exist, would be a class of potential falsifiers.

Mind-body interaction results from complexes of many states, processes and systems

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. Accordingly, we may understand a human's eventual decision to travel across the world to meet a long-lost relative as being the product of many distinct states, processes and systems over an extended period of time - including unconscious states and processes, such as in dreams, and including World 3-based knowledge, for example regarding available modes of transport and necessary documentation and immunisation.

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 (e.g. many aspects of visual processing may result from a body-mind parallelism, which may explain why knowledge of an optical illusion still leaves us helpless to visualise any differently). But, considered in terms of biological purposes, Popper argues that it does not fit with an evolutionary view of the mind to consider the entire plethora of 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'.

World 3 brings an additional 'system' into understanding interaction of the human mind and body

Popper's theory of World 3 takes the last argument a step further, by regarding the human 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, and grow our knowledge beyond what is otherwise 'built-into' brain and mental states/processes, by having our knowledge advanced through non-violent, non-lethal intellectual competition and co-operation - centred on the creation and evaluation of different kinds of World 3 object and "objective knowledge". 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.

Mental activity as the bridge between World 1 and World 3 and as an agent of their evolution

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 the different kinds of interaction within World 1 (e.g. the different physical forces with different sets of particles; the effect of structures on their components and vice versa); and also for how a 'train of thought', or any other mental process, is causally constituted within World 2. 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, in the sense under discussion, of how their theorised 'interactions' are causally possible:- 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, yet 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.

Popper's account of mind-brain interaction therefore eschews what he regards as either sterile or irresolvable causal issues (e.g. as to how the causation involved is possible or actually occurs) to emphasise that we nevertheless have sufficient reason to conjecture that such interaction does occur. He argues that the mind-body interface is one where the relevant World 1 and World 2 states/processes are highly active - intensely alive - and that mind-body interaction should be understood as resulting from intense live activity rather than lifeless causal mechanisms; and he suggests the ways the mind 'reads off' from brain activity follows the active model of "making and matching" that is central to how knowledge develops within the tiny fraction of World 1 that is alive.

In Popper's later work he proposed a 'spearhead' model for Darwinian explanation in line with his three world's theory:- supplementing the view that 'natural selection' works on passive genetic material with the view that the activity of organisms (e.g. in pursuing new ecological niches) may crucially change the character of their selection pressures. Where such changed selection pressures emerge from changed behaviour due to an organism's World 2 preferences and/or mental states/processes, we may see both the immense evolutionary value of World 2 and its immense role in affecting evolutionary change in World 1 terms:- (a) how World 2 activity can act as a 'spearhead' for behavioural change and consequent gene-pool change within World 1; and (b) how the increased variation in behaviour facilitated by World 2 greatly increases the variety for 'natural selection' to cull, and so provides a much richer field for a theory of evolution by 'natural selection' than the field of genetic mutations alone. In a similar way to how World 2 may enrich World 1, for example by enlarging the repertoire of behavioural responses within World 1, so Popper argues that World 3 also enriches the human World 2 and the possibilities for human action in World 1. In both cases of interaction - World 1 with World 2, and World 2 with World 3 - Popper emphasises the active role of World 2 in producing evolutionary change within World 1 and within World 3.

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]

Notes and abbreviated references

^ Popper 1968.
^ Eccles 1970, p. 163–169.
^ Heller 2011, p. 118–120.
^ Niiniluoto 2006, p. 61: "The most important difference between Platonism and Popper is the fact that world 3 is created by human beings."
^ Eccles 1970, p. 165: "Most important components of world 3 are the theoretical systems comprising scientific problems and the critical arguments generated by discussions of these problems."
^ Eccles 1970, p. 165: "Popper specifies for these three worlds, namely that there is reciprocal transmission between 1 and 2 and between 2 and 3, but that 1 and 3 can interact only by mediation of World 2."
^ Popper 1972, p. 155: "The first world and the third world cannot interact, save through the intervention of the second world, the world of subjective or personal experiences."

References

Eccles, J. C. (1970). Facing Reality: Philosophical Adventures by a Brain Scientist. Springer Science & Business Media. ISBN 978-1-4757-3997-8.
Heller, Michael (2011). Philosophy in Science: An Historical Introduction. Springer.
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.

v t e Karl Popper
Philosophy	Bold hypothesis Critical rationalism Falsifiability Growth of knowledge Open society Popper's experiment Popper's three worlds Verisimilitude
Works	The Logic of Scientific Discovery (1934) The Poverty of Historicism (1936) The Open Society and Its Enemies (1945) Conjectures and Refutations (1963) Unended Quest (1976) The Myth of the Framework (1994)