The Existence of the Physical World
Most people idly wonder, at some point, whether the physical world exists, or whether it is all one great illusion. We could be under the sway of a deceptive demon, or (in a more modern alternative) brains in the vat of some mad scientist, or (in an even more contemporary version) stuck in The Matrix. When it comes time to have a cup of coffee, talk to a friend, or even do some work, we tend to put these possibilities out of our heads. But we might wonder whether doing these things has any point if there is no physical world. If we can't prove that there is, are we justified in thinking and acting as if there were? And what does it mean to wonder whether there is a physical world, anyway?
The last question is, of course, the first that we have to consider. It is actually tougher than we might suppose. The common-sense answer is: a three-dimensional world filled with objects, each of which has a certain colour, shape, smell and so on. This view seems simple, but (as we shall see) it is actually quite mysterious and ultimately untenable. A less ambitious answer is that the physical world is something external to us, at least in part causing our sensations, ideas and so on. This must be a part of the notion, but it cannot be the whole of it. For many 'idealists', who have denied a physical world, nevertheless think that there are some things external to us - for instance, other minds, or Berkeley's ideas in the mind of God, or Leibniz's rudimentary minds.
Material things must then be both external to us and not mental. It is harder to get a more precise grip on the notion of 'material' than that, which may bode poorly for it. Often, when we cannot articulate a notion, and only say what it is not, this is because it is not really reflective of anything.
Before we go on to examine this, someone might want to ask whether we really can assume there is an external world. Perhaps all the places we pass through and people we talk to are just figments of our imagination, as in a dream. This position is known as solipsism, and it really would alter the way you thought about the world, placing all the objects and people (other minds) that we interact with solely within your own mind. It is important to realize that it cannot ever be completely disproved: any evidence could be dismissed as the invention of a bloody-minded mind, intent (for some obscure reason) on deceiving itself.
While solipsism cannot be disproved, most people find certain considerations against it persuasive. The (subtly different) questions of whether these considerations would convince the strongest sceptic, and whether they should convince us if not, are dealt with in another essay. Here, I shall assume that our judgements are reasonable, seeing only where they lead us.
What convinces us (rightly or wrongly) of the falsehood of solipsism is that we are not conscious of creating the world around us, but often seem to passively experience it, encountering new things (like elephants) we had never come across before. The idea of an elephant could admittedly be fabricated from pre-existing concepts, but some of these (like the colour grey) also come from experience, and could not. It could always be argued that these are creations of the unconscious mind, but this is something external to our immediate consciousness, 'discovered' by science and psychology rather than any direct awareness of it, so would strictly count as part of an external world. We can thus convincingly argue that the theory that something external affects our mind fits much better with our everyday experience.
So if we say that our experiences likely come at least in part from something external to us, where do they come from? What about the possibility that there is an external world, which causes our perceptions and ideas, but it is completely unlike the world we assume exists? This is the case in Descartes' famous supposition that an evil demon might be feeding him false perceptions. It is interesting to note that in the common thought experiments of the brain-in-the-vat, The Matrix, and the evil demon (if you think demons are physical, that is - Descartes spoke of a 'spirit'), there is a physical world which causes our experiences; we are just deceived about the contents of that world. But when we talk about the physical world existing or not, we tend to mean a physical world somewhat like the one we seem to experience.
Again, these possibilities cannot be completely disproved. This is because we can only know about the physical world through our senses and what we learn from them. Descartes thought that he could banish the evil demon altogether by reasoning a priori, without reference to experience. His attempt to do so, by first proving God's existence and then arguing that since God is no deceiver, the physical world must exist, is largely discredited. But his general a priori approach was also misconceived, as the physical world is simply what we seem to perceive through our senses, so cannot be known a priori.
But several things convince us that we are not being deceived about the external world. For a start, it seems an entirely superfluous suggestion. The world seems quite unconcerned with us, or any conscious life - so far as we can tell, the laws it obeys are entirely impersonal. Sometimes they favour us, sometimes they don't; we seem to be accidental byproducts of nature. Granted, this appearance could all be part of an extraordinarily clever, apparently purposeless, illusion, but it is reason enough for supposing that the world around us is probably in some way real.
As mentioned earlier, several idealists have come up with worldviews that accept an external reality that we perceive, more or less correctly, but account for it purely in terms of mind, not matter. One of the most famous systems of this kind was that of Bishop Berkeley, an 18th century Irish philosopher. Like the solipsists, Berkeley noted that all we are ever directly aware of are the ideas in our mind (some philosophers would disagree with him, but this exploratory essay is not the place to dismiss their arguments). From this observation, he tried to show that it makes more sense to suppose that there are only ideas in the world than to suppose that there is matter, too. If we believed in matter, we would only be positing the existence of some mysterious, totally hidden substance, divorced from anything that we experience.
And this would violate Occam's Razor. Berkeley also exploited the confusion over how physical events could cause states of mind, arguing that they required mental causes. J.L. Mackie illustrates this argument with the analogy of a billiard ball moving because it is hit by another billiard ball, not because of some unrelated event halfway round the world. The implicit principle is that an event requires a similar cause, so physical causes could only give physical events. Modern neuroscience to some extent undermines this by revealing a close correlation between the physical and the mental, which are clearly not so separate as they were once thought to be.
Berkeley's simpler account of the world was that things exist only in being perceived: the Latin slogan was esse est percipi. However, he avoided the claim that the world ceases to exist when we are not aware of it (the weakness of solipsism.) Objects continue to exist, independent of our perception of them, by virtue of being ideas in the mind of God, which are what God makes us aware of when we experience the real world. So there is an external world, found in God's mind, just not a physical one. Matter is dispensed with altogether.
There are, however, great difficulties with explaining the world in purely mental terms. As Bertrand Russell pointed out in The Problems of Philosophy, though ideas and perceptions are by definition in our minds, that does not mean that their objects are. For instance, idealists generally accept the reality of other people (or at least other minds), yet when we 'bear them in mind', that does not mean that they only exist in our minds any more than the fact that the idea of an orange can only exist in your mind implies that there is no orange outside it. There seems to be an inconsistency here.
The metaphysics behind Berkeley's system has since been undermined. If all the traditional arguments for God's existence fail, and we see no reason to believe in any sort of 'divine mind', it is difficult to see a mental basis for facts about the world. This either takes us back to the problematic view that cats and the rest cease to exist when we are not around, or requires that they have a non-mental existence. And by our definition, a non-mental existence is a material existence.
At this point, we must return to the question of just what material existence is. We know what mental existence is, because our experience of our own thoughts and perceptions is as direct as one could want. But we can only experience the physical objects we take to exist through the medium of such perceptions, so strictly speaking we only know about their power to cause these. This raises the interesting question of whether we directly perceive the external world, or are aware only of our minds' representations of it. And, if the answer is the latter, can we have any idea what the physical world is like?
Locke and the British empiricists in the early centuries of the scientific revolution made a famous distinction between 'primary' and 'secondary' qualities of objects. Primary qualities are the basic elements of modern physics: extension (shape), solidity and motion. Secondary qualities are things like colour, taste and smell, which are a major part of our perception of the world but can be understood in terms of the primary qualities that cause them. For instance, modern science tells us that colour can be understood by the propensity of a surface to reflect certain frequencies of light, dependent on its structure. So an orange doesn't really 'have' a colour, only the ability to cause the experience of orange in a mind.
By itself, this is a radical alteration of the way we look at the world: every object we see appears coloured, so to that extent we are not seeing the world as it 'really' is. Unreflective common sense tends to assume that oranges really are orange. Yes, they may look different given different lighting, perspective or eyes, but there is one right colour: the one you would see in bright sunlight, with normal human eyes. But, as Russell pithily observed, this is "favouritism".
This implies that the physical world is made up of objects with shape and solidity, in motion or at rest, and that colour and other 'secondary' qualities are just representations of these basic, 'primary' properties. However, even this view is extremely problematic. We can understand the primary qualities only by considering how they affect us, the observers. The shape of an object, for example, looks different from different perspectives, none more privileged than any other. So the three-dimensional shape we attribute to it cannot be directly perceived; is is constructed out of these two-dimensional perceptions.
And when we consider what it is that fills this shape, what it is that moves around, we are left able only to say that it is something 'solid'. Locke was very taken with this quality, but even his description reveals that it can only be understood in terms of the resistance solid objects offer us. Berkeley argued that this meant we were back to understanding the object solely in terms of our perceptions of it, which can differ from creature to creature: an orange will offer more resistance to an ant than to a woodpecker.
So we experience objects purely through their capacity to cause certain sensations in us, or alter other objects, which in turn cause sensations in us. This conclusion sounds tautological once we have laid it out: how else could we experience them? We have seen that there is good reason to suppose that objects have an external, non-mental existence. But we seem now to have concluded that we can have no understanding of what this existence is except by reference to our internal mental states. Imagine an orange losing its potency to cause various experiences one by one. First, its taste goes, then its colour, then its smell, then the sound it makes when bounced. Finally, with the sense of touch, we lose the last manifestations of what we call its shape, as well as its texture and resistance. We now seem to be left with no concept of what the orange is, in itself.
We may be reminded of Kant's claim that we can know nothing of the real, 'noumenal' nature of objects, and our limited to considering the phenomenal world of our experience, which is dependent on the mind for its structure and appearance. One of Berkeley's main arguments against those who believed in matter was: "you know neither what it is in itself, nor what relation it bears to accidents." If this is really the case, we may be worried about what it means to say the physical world exists.
The picture of the world that modern physics offers is that of a vast, interrelated network of forces, operating on a mysterious thing called matter. This can be seen in the way matter is merely defined as "the object of inertial mass", which says nothing about what it is in itself. Scientists would doubtless dismiss a request to do more as meaningless.
However, what physics does offer is a powerful, coherent and elegant way of understanding why we have the perceptions we do, or - if your inclinations are anti-realist - of systematising them. For instance, the different resistances oranges offer to woodpeckers and ants can be understood in terms of woodpeckers' greater force and sharper beakes, letting us assign a constant number to the amount of resistance offered by our orange. Likewise, all observers in scientific enquiry can agree that oranges reflect those frequencies of light labelled 'orange', no matter how such frequencies appear to their different eyes. Similar stories can be offered for all the other secondary qualities. Finally, physics treats matter as susceptible to certain apparently constant physical laws: oranges interact with other objects we perceive in predictable ways. So positing its existence goes a long way to explaining our experience.
This view of matter as a theoretical construct brought in to explain our experience is admittedly a long way away from our ordinary picture of the world. It treats our perceptions of the world not as neutral representations of the 'real' structure of things, but as to a large extent reflecting the structure imposed on that world by our minds. The similarity this picture bears to Kantianism is readily apparent: our knowledge of the world can only come from its effects on the five senses of sight, hearing, smell, touch and taste, while any 'noumenal' nature it might have remains forever hidden. Our senses have evolved to give us what we need to survive, not to satisfy our desire for knowledge of how the world really is (a desire which even if it is intelligible. We should be thankful that they seem to do at least that.