Essays | Books

Does the External World Really Exist?

On what grounds are we justified in believing in an external world?

Is the external physical world as we perceive it or is the true nature of reality very different? Indeed, are our perceptions caused by an external world at all – could it be that the world is merely a convincing simulation created by a supremely intelligent and manipulative demon? Perhaps nothing exists except our mind and the demon. In this essay we shall question our common sense belief in an external world. Following Descartes ‘method of doubt’ we shall systematically destroy the foundations for this belief to arrive on the bleak, featureless plain of hyperbolical doubt. We shall then attempt to claw our way back to an objective reality by examining objections to the sceptical conclusion. The journey will raise questions about the existence of other minds and the very language we use to express our scepticism.

What do we mean by ‘the external world’?
By ‘external world’ we refer to the totality of objects and phenomena which exist independently of our mind and which are revealed to us through perception (sense data). This is the domain for which science attempts to provide a causal explanation for how the objects of the world act on each other and on our senses. We have no direct contact with the external world but derive knowledge of it through rationalisation of the patterns in the sense data (qualia) we experience. We hypothesise that our senses reveal a world of objects which have extension in space and time and which have causal relationships with each other and, through our senses, with ourselves. Our rational minds correlate and classify sense data into abstract classes and concepts, identifying instances of known classes or combining classes into composite concepts which may or may not exist in some external world (unicorns for example). Our mental model is intentional; containing tokens which are ‘about’ objects in the real world, linked by logical relations which (we hypothesise) are isomorphic to causal relations between the corresponding objects in an external world.

But do we have any rational grounds for believing that there is in fact any external reality outside our own mind? What can we know with certainty and what must we doubt?

What is immune from doubt?
In his Meditations Descartes investigates the possibility of knowledge by asking whether we can separate out what is solid and what is unreliable in our system of commonsense beliefs. His ‘method of doubt’ asks us to treat as false anything in which we can entertain the slightest possibility of doubt. Descartes’ argument progresses through three successive stages of increasing scepticism.

Descartes firstly questions the evidence of his senses in the Argument from Illusion. Given that he has been deceived in the past, how can he trust any sensory impressions at all? This he qualifies by accepting that the corroborative evidence of several senses using different evidence gathered in good conditions is sufficient to establish a proposition (such as the stick in the water being straight and not bent). I can be deceived by my senses some of the time, he reasons, but not all of the time.

Next Descartes asks whether all our experience could merely be part of a dream. My impressions while dreaming are in most respects identical to the same experiences while awake. In my dreams I can experience unreal objects, such as unicorns, which appear to me as both real and entirely consistent with the network of my other beliefs. How can I be sure that my waking experiences are not similarly ideas within another level of dreaming and are not grounded in any objective reality? Clearly though, I have the complementary concepts of dreaming and wakefulness together with clear recollections of both dreaming and waking. While I can be deceived while asleep, I cannot doubt all my perceptions since I know that I am sometimes awake. The objects of my dreams must derive from experience while awake. Furthermore, arithmetic and geometric truths are valid whether I perceive them dreaming or awake.

Surely though, we cannot doubt the present contents of our mind - we have perfect and privileged knowledge of them. To doubt our immediate mental states requires an independent position from which the mind can view both itself and its own perception of itself in order to validate the correspondence. We cannot “jump out of the system” without resorting to an infinite regress of homunculi; a hierarchy of meta-minds where each homunculus observes the mental states of the homunculus one level down. This merely pushes the problem to another level without any ultimate resolution. There is a parallel here with Godel’s revelations about the inability of systems of mathematics to prove that they are themselves complete and consistent without the use of axioms that are outside the system being inspected. If I say “I doubt that I am in pain” then surely what I doubt is the meaning of the word “pain” and not whether I am experiencing a specific mental state. [We will not discuss here Wittgenstein’s elaboration of this in the Private Language Argument]

Mathematical and logical truths must also be immune from doubt. Abstract systems such as number theory, Boolean logic and Euclidian geometry are entirely coherent and meaningful in themselves without the need for concrete instantiations in a physical world. Two plus two still makes four whether I am asleep or awake or whether my mind is inside a head or floating in the ether. An interesting point here is whether such logical and mathematical truths are innate concepts (as Plato believed) or whether they can only arise through experience of their analogues in a real physical world (as Locke believed). For example, we could form our concept of number and counting only by abstraction from experience of singular and multiple instances of objects in a real world.

What must I doubt?
Accepting that I can be certain about my current mental states, how certain can I be about the cause of those states? Indeed, do my mental states have a cause outside of my mind? My sense impressions appear to make sense if interpreted as representations of objects which have extension in an external space and time, which interact with each other according to regular causal laws and which have properties which I can perceive and measure. My sensory states could be caused then by a real external world that impinges on my sense organs which in turn cause brain states upon which my mental states supervene. But is this the only possible interpretation of our experiences?

Descartes’ final move is to recognise that the mind is a closed system which appears consistent to itself but which lacks an external vantage point from which the system as a whole can be verified. As Bertrand Russell put it, the only thing we ever see is the inside of our own heads. Descartes realised that since the mind only has privileged knowledge of our senses, there are no rational grounds for believing in an objective physical world. Our experiences would be entirely consistent with there being an external demon who manipulates our senses in a way that produces an entirely convincing simulation of an external world. Similarly, while there could be an external world, its nature may be entirely different from our conception of it due to distortion at the mind-body interface. Because we can only investigate the nature of the world through a part of the world itself, the truth of the situation is impossible to resolve by empirical methods. Any scientific experiment designed to reveal the nature of reality is mediated through the same suspect channel that is the very subject of our doubt.

Is the objective world a fiction?
One conclusion is that, since we cannot distinguish between the reality of an external world and a fiction created by an evil demon, then the external world is indeed a fiction. Indeed, if the evil genius can delude me about the external world than can I not also be deceived about my use of logic and arithmetic? Perhaps I am systematically deceived into thinking that two plus three is five whereas in reality they make six? Descartes is now driven into a corner where the only certainties he is left with are the existence of his own thoughts (and hence himself, via cogito ergo sum) and his ability to use language to reason about his existence. Even in the depths of radical doubt we must cling to a belief in reason, for without it neither scepticism not its opposite are coherent concepts – we have absolute darkness.

But if our beliefs about the causes of our experiences could be false, then we must even doubt the necessity that our experiences have any cause. Even the hypothetical demon can be doubted since there are no rational grounds for believing that our thoughts require a cause outside of themselves.

Can the external world really be doubted?
The sceptical conclusion can be attacked on a number of grounds. To apprehend the true nature of reality we need to escape from the prison of our first person perspective. It is the rationalist quest to seek a viewpoint from which we can see the whole of totality - both the real world and our place within it. It was Kant’s conclusion that such a perspective is unattainable - our perceptions are the limits of our perspective and hence the limits of thought and hence knowledge of the world. To Kant, ultimate knowledge of the world outside the limits of our mind is forever outside our grasp.

Descartes himself resorted to belief a veracious God who must exist necessarily by definition (the so-called ontological argument). A guileless God restores objective reality to Descartes – any error must be attributed to Descartes’ own mis-interpretation of God’s truthful revelation. But surely this is not a satisfying resolution to the problem. For a start, the ontological argument for God is unsound for it relies on perfection (i.e. God) requiring by necessity the attribute of existence. Secondly, does not Descartes’ argument rely covertly on the pre-existence of an objective world for a truthful God to reveal to us? This is the so-called ‘Cartesian circle’ for which most commentators doubt Descartes had a convincing answer.

It could be argued that the belief in an external world is justified by inference to the best explanation. But is it valid to object to the evil genius merely because it seems manifestly improbable? After all, it provides an equally good explanation of my experience. Surely a reality composed of just two entities (my mind and the demon) is more likely than a physical world requiring a myriad of objects which interact in ways we barely comprehend? With no certainties to rely on I have no grounds for believing what is improbable and what is not – there is no objective ground from which to judge the merits of competing explanations since they present identical evidence whose cause is beyond the scope of empirical investigation.

A more convincing response is provided by Wittgenstein’s transcendental argument. This claims essentially that in order for us to use a language to express statements about external objects, then the objects of those statements must necessarily exist. Donald Davidson and Hilary Putnam raise related arguments from interpretation and from reference respectively. In order for Descartes to reason about his scepticism then he must be certain about his ability to use language to describe concepts such as certainty and the self. Wittgenstein’s ‘private language argument’ shows that this cannot be a language private to Descartes he could never be certain that he is using it correctly. This language must be a public language whose correct usage is validated by a community of other language users (co-players of a shared language game.) For Descartes to think about his thinking then he must be using a language which is part of some public realm in which others can lean and use the language. To doubt that ‘this is a hand’ presupposes the ability to use the term ‘hand’ correctly according to the rules of a language game played with others. Without an objective public community Descartes’ language, and hence his argument for scepticism, would have no meaning. In other words, I can only question whether there are other language users if I use a language shared by others. Hence radical scepticism must be false. Wittgenstein’s argument effectively moves the problem from ‘can I have knowledge?’ to ‘can we have knowledge?’.

Wittgenstein claims that Descartes’ refuge from the evil demon in the certainty of private knowledge of his mental states is a falsity for the demon can also deceive him in his use of language. To allow Descartes to make any form of rational argument for or against scepticism we must admit the existence of an objective world in which the very language of reason can be learned. In other words, no first person perspective on the world is coherent since it leads to radical scepticism. Only by adopting a third person perspective can we correctly perceive our place within an objective world and ultimately defeat the demon. But even this approach is not conclusively immune from the possibility of the evil demon - could not the demon conjure a ready-made community of language users within the mind of the sceptic?

Daniel Dennett replies to Descartes in characteristic style by providing an evolutionary argument. This is based on the materialist view that our minds supervene on brain states within material bodies which are the product of evolutionary selection. Our rational model of the world governs our behaviour in that world. In humans this is more than mere conditioned reflex - we have evolved a consciousness which perceives ourselves as objective entities within the world. Such minds have only evolved over many generations by virtue of being able to survive and develop in a dangerous world. Since we act according to our beliefs, our mental model of the world must bear close correspondence to external reality for this model to afford our survival. If appearance and reality we very different then, argues Dennett, our beliefs would quickly lead us over the nearest cliff. Because we are rational creatures we can pre-execute our strategies in our heads and allow our ideas to die in our steads. Our conceptual external world must therefore be grounded in an objective reality.

This argument must of course pre-suppose a number of key concepts which cannot be immune from the evil demon. Dennett here presupposes that the mind exists by virtue of a physical body that inhabits a physical world where selective evolution can take place. Given this starting premise then the argument is cogent but signally fails the address the root problem – can we justify a physical external world in the first place?

We have shown that, at first sight, almost nothing is immune from doubt and hence could be false. Our common sense belief in the nature of the world has no rational grounds and could, as Descartes has shown, be replaced by any number of alternative explanations which yield the same subjective experience. At the very least however we must preserve certainty of our own thoughts and of our ability to reason about the causes of those thoughts. It is this very ability to reason within a structured language which has rules of correct usage that offers the self-verifying justification we seek. To express my doubt about your existence I must explain myself in terms that you understand. According to Wittgenstein this is only possible if you, and by extension the rest of the objective world, actually exists.


1. Rene Descartes, Meditations of First Philosophy, 1
2. Ludwig Wittgenstein, On Certainty, Blackwell, Oxford 1969
3. Ludwig Wittgenstein, Philosophical Investigations, Oxford 1952
4. Hilary Putnam, Reason, Truth and History, Oxford 1984
5. Immanual Kant, Critique of Pure Reason