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The aim of this article is to exam several modern philosophers’ (Descartes, Berkeley, Hume, and Kant) view on the ‘external world’. In attempting to illustrate such a diverse problem, it is wise for us to only look at a few key representations on the issue. To give our problem a fair treatment, I’ll also include several objections against each philosopher’s theory of the external world.

The ideas presented in dualism can be traced as far back as Plato, but it was Rene Descartes who made it famous. In his autobiographical work the discourse, Descartes explained how he had failed to find truth in both the academic schools and the common people of medieval Europe. Unsatisfied, he set out to search for truth within himself, his quest for truth ultimately resulted in the Meditations. The main concern of the Meditations is to uncover an absolutely indubitable starting point as the foundation for knowledge that could stand against the raging skepticism of his time. Many philosophers are willing to settle for less, but Descartes’ training in mathematics and logic shaped his philosophy in such a way that demands absolutely sound axioms for his starting principles (which is very similar to mathematics and logic). Out of many arguments presented in the Meditations, we’re here mainly concerned with his argument for the distinction of primary and secondary qualities. To quote Descartes:

‘Take, for example, this piece of wax; it is quite fresh, having been but recently taken from the beehive; it has not yet lost the sweetness of the honey it contained; it still retains somewhat of the odor of the flowers from which it was gathered; its color, figure, size, are apparent (to the sight); it is hard, cold, easily handled; and sounds when struck upon with the finger. In fine, all that contributes to make a body as distinctly known as possible, is found in the one before us. But, while I am speaking, let it be placed near the fire–what remained of the taste exhales, the smell evaporates, the color changes, its figure is destroyed, its size increases, it becomes liquid, it grows hot, it can hardly be handled, and, although struck upon, it emits no sound. Does the same wax still remain after this change? It must be admitted that it does remain; no one doubts it, or judges otherwise. What, then, was it I knew with so much distinctness in the piece of wax? Assuredly, it could be nothing of all that I observed by means of the senses, since all the things that fell under taste, smell, sight, touch, and hearing are changed, and yet the same wax remains. ‘

Obviously the wax continues to be despite the annihilation of all of its sensible features. People would only say that the taste, smell, color, sound and touch of the wax are changed, but the essential thing that is the wax remains the same. From this, Descartes concluded that the sensible features of the wax must not be essential to the wax and he proceeded to find out what constitutes the actual wax. Upon the first examination, Descartes found out that we could say the wax take up space. But what does it mean to say something take up space? How much space does the wax take up? Descartes quickly changed his answer and claimed that one of the things he was certain about the wax was that it’s extended in general (as opposed to any specific amount of extension). Upon this conclusion, he successively found out other qualities that are essential to the wax, namely: flexibility (its size and shape are changeable) and mutability (its sensible qualities are changeable). What does Descartes’ conclusion imply for the nature of the external world? The answer is simple, the nature of the external world is not of sensible qualities (color, taste, etc), but are of intelligible qualities (extension, flexibility, etc). Boyle clarified Descartes’ conclusions by calling the former secondary qualities and the latter primary qualities. In other words, the essential nature of the external world consists of primary qualities while the secondary qualities are merely accidental. By this conclusion, Descartes also laid the foundation for modern physics. If the nature of the external world consists only of the primary qualities, then we could use mathematics to represent their values (using numbers, figures to represent size, shape, velocity, etc). An interesting note is that Galileo hold a similar position with Descartes, quote:

‘I think that tastes, odours, colours, etc., are nothing but mere names for something that resides exclusively in our sensitive body, so that if perceiving creatures were removed, all of these qualities would be annihilated and abolished from existence.  But just because we have given special names to these qualities, different from the names we have given to the primary and real properties, we are tempted into believing that the former really and truly exist as well as the latter.’

Unfortunately, he never followed through with his theory. The dualists’ view of the external world is purely mechanical. They claimed the cause of our sensations (secondary qualities) lays in something unobserved (primary qualities). In this way, the external world became an unthinking machine that follows the various laws of nature. They reconciled our abilities such as thinking and imagining with the unthinking world by placing the former qualities in another category (category of the soul/spirit). To our present purpose I’ll not go any further into that aspect of dualism. The conclusion of Descartes seemed plausible enough, but its inherent flaw ultimately bankrupted the theory. Descartes could not account for the operations between the soul and the physical world. A mechanical explanation ends somewhere in the pineal gland for Descartes, but he could not fully explain how then, the pineal gland would go on to supply the soul with colors and such.

George Berkeley, the first idealist thought to end the religious unrest of his time by denial of the existence of material objects. As history would show, the reformation in Europe increasingly stressed the Catholic schools to come up with an academic attack on their opponents. One form of the arguments introduced against the reformers was skepticism (they looked back to Pyrrho and Sextus Empiricus); unfortunately this had proven to be a double edged sword. Soon, the reformers begin to use skepticism to attack back, which ended in outright skepticism and religious unrest all over Europe. Descartes thought to combat skepticism by finding something indubitable; however his efforts only lead to more skepticism due to the problems of his theory. Descartes’ theory even implied that the world doesn’t need a God and can be entirely naturalistic. As Descartes’ theory failed to account for the problem of our soul’s interaction with the physical world, more and more people turned to the naturalistic part of his theory (take away the soul part) and ended in atheism. Berkeley thought the problem of atheism and skepticism was due largely to what the philosophers called material object or matter. He thought that the term material object is merely confusion on the philosopher’s part and could be show, with arguments, to not exist. Berkeley started his argument by claiming the object of our sensory experience consists of either ideas of sensation (five external senses) or ideas of reflection (internal sensations, such as anger, sadness, etc.). Things we claim to exist in the world around us are in fact collections of ideas. They may be imagined (internal) such as a unicorn, or sensed (external) such as a table, but they are all ideas. Berkeley concluded that ideas can only exist when they are perceived and they can only be perceived by minds. Quote:

‘That neither our thoughts, nor passions, nor ideas formed by the imagination, exist WITHOUT the mind,

 is what EVERYBODY WILL ALLOW. And it seems no less evident that the various sensations or ideas imprinted

 on the sense, however blended or combined together (that is, whatever objects they compose), cannot 

exist otherwise than IN a mind perceiving them. I think an intuitive knowledge may be obtained of this by any 

one that shall attend to WHAT IS MEANT BY THE TERM EXIST, when applied to sensible things. The table I write 

on I say exists, that is, I see and feel it; and if I were out of my study I should say it existed—meaning thereby 

that if I was in my study I might perceive it, or that some other spirit actually does perceive it. There was an odour, that is, 

it was smelt; there was a sound, that is, it was heard; a colour or figure, and it was perceived by sight or touch. This is all that

 I can understand by these and the like expressions. For as to what is said of the absolute existence of unthinking things 

without any relation to their being perceived, that seems perfectly unintelligible. Their ESSE is PERCIPI,nor is it possible 

they should have any existence out of the minds or thinking things which perceive them.

It is indeed an opinion STRANGELY prevailing amongst men, that houses, mountains, rivers, and in a word all sensible objects, have an existence, natural or real, distinct from their being perceived by the understanding. But, with how great an assurance and acquiescence soever this principle may be entertained in the world, yet whoever shall find in his heart to call it in question may, if I mistake not, perceive it to involve a manifest contradiction. For, what are the fore-mentioned objects but the things we perceive by sense? and what do we PERCEIVE BESIDES OUR OWN IDEAS OR SENSATIONS? and is it not plainly repugnant that any one of these, or any combination of them, should exist unperceived?’

Berkeley’s theory is also in part a reaction against Newton’s Principia, which continued Descartes’ machinist philosophy by claiming the world consist of primary qualities. Berkeley attacked the distinction by arguing that the primary qualities cannot be separated from the secondary qualities. This is also an attack on abstraction and the abstract in general. Whenever we use a term such as human or triangle, we’re not talking about anything that exists. Try to picture human in general (that is tall and short, dark and light, good-looking and not) and triangle in general (being both equilateral and isosceles, both right and acute), it cannot be done. It’s obvious we can’t give a name to every particular thing we come across, as human memory would not allow it. We therefore took the shortcut of grouping several things with similarities together and give them a general name in regard of their similarities. The general man does not exist, but the term exists and can refer to any specific man that share characteristics with other beings termed man. Therefore the primary qualities such as motion in general or extension in general cannot be conceived, we always conceive some specific thing having motion and extension. Therefore primary qualities are not essential to any object and are just like secondary qualities – depend on the perceiver. Now a natural question to ask is what caused those ideas that we perceive? Berkeley’s answer is God; God caused all of our ideas. This follows because an idea can only exist if it’s perceived, but it’s absurd to think ideas would go in and out of existence based on our observation, God’s mind perceives all ideas and therefore secures their existence. The external world to Berkeley consists of ideas, which are in the mind of God.

David Hume followed the empiricist tradition to its logical conclusion, which ended in totally skepticism. Hume accepted Berkeley’s attack of primary and secondary qualities, but he did not accept Berkeley’s God. For Hume, there is nothing securing the consistence of our ideas at all (for Berkeley, God did). To quote Hume:

‘We may well ask, What causes induce us to believe in the existence of body? But it is in vain to ask, Whether there be body or not? That is a point, which we must take for granted in all our reasonings. ‘

Further analysis of this passage revealed two connected questions: ‘Why do we attribute a continued existence to objects even when they are not present to the senses?’ and ‘Why do we suppose them to have an existence distinct from the mind and perception?’. Before we go any further, I must point out that Hume is only dealing with our perceptions of objects, not with the nature of objects themselves (as Hume didn’t believe we could know the nature of external objects). Those two questions are really questions regarding the continuance of objects through intervals of non-perception, and their independence of the perceiving mind. There are many examples in day to day life that can lead us to those questions. When we see an object and blink, the blink creates discontinuance in our perception of that object. Yet despite that, we still hold firm in our belief of the continued existence of the object. It’s not just blinks, we sometimes look away from the object or we fall asleep and lose our perceptions all together. However, our belief in the existence of those objects doesn’t just stop. Our sense-experience is not fragmentary, but continuous, at least while we’re awake. But this doesn’t answer Hume’s questions. It’s true that while we’re awake, we experience some sort of sense-perception. Those sense-perceptions are only continuous in so-far as they are not interrupted as a whole, particular object in our sense-perception is not continuous in the same sense. To use a real life example: when I see a cup then blink or look away, the sense-perception of the cup is discontinued, however briefly. During the absence of the cup sense-perception, we still hold the belief that the cup would continue to exist and not vanish when we lose its perception. We commonly believe that the interruption of our sense-perception is only an interruption in our observation but not in the existence of the actual object. Hume is concerned here with what lead us to belief in the existence of objects without sense-perception. He put forward three possible answers in the Treatise. They are: sense, reason, and the imagination. It is simple and obvious that our belief in the continued existence of the cup did not come from our sense-perceptions, as that is precisely what we lack. It also can’t come from reason; Hume argued that if our belief did rise out of reason then only a few learned philosophers would have knowledge of that reason. But as it stand, everyone seem to believe in the continued existence of bodies, it seems that even animals have this sort of belief. Now it is obvious most people can’t produce any reason for their belief in the continued existence of bodies, and yet they still hold that belief. Therefore reason can’t be the cause of our belief in the continued existence of bodies. At this point, Hume gives us his own answer, which is the only option left, namely the imagination. To explain this in more detail, the particular characteristics that caused us to believe in the continued existence of bodies are two in number. They are constancy and coherence. To quote a Hume scholar:

‘It is simply this: that when we observe a thing again, after an interval during which we have not observed it, we often find that it has the same sensible qualities and relations as it had before.’

To put it more abstractly, we have a continued series of similar sense-perceptions A1, A2, A3, A4, then at some point the series is interrupted blank, blank, and continues at A7, A8, and so on. Constancy is a combination of the characteristic of ‘similarity, continuity, and interruptedness’. In other words, constancy is a closely resembling series of sense-perceptions with a gap in between its ends. Our example of the discontinued sense-perception of the cup would be an example of constancy. Coherence is a little different. To put this abstractly, we have a series of impressions A, blank, blank, blank, E, … While the things in this series do not resemble each other, we’ve on previous occasions perceived events in this order. To use an example, I see a dog walking along a stretch of sidewalk from point A to E. While looking, I turn away for a few seconds to pour water in my cup. Now the dog is at point E but I’ve not seen it walk there. However, I have, on many previous occasions seen dogs walk from point A to point B to point C to point D and to point E, the whole series. In the present case my mind makes the connection between A and E despite having not seen the in-between B C D. And coherence is just this, our mind’s ability to connect interrupted series of dissimilar impressions

What could account for this apparent paradox? For Hume, the idea of ‘time or duration’ is the only answer. To quote a Hume scholar (again):

‘‘The phrase ‘identical with’ stands for a relation, and it must hold between two terms at least. Where there is only one term, there is unity but not identity. On the other hand, if there are many terms, we cannot but admit that they are numerically different, however much they may resemble each other.

Consider any entity which remains absolutely unchanged throughout a finite period of time – ‘any unchangeable object’ as Hume rather oddly calls it. We say ‘it is the same as it was two minutes ago’. Now strictly speaking this is not true. Indeed, it is not even sense. For since our object has not changed at all, we cannot distinguish any multiplicity of successive stages of phases within it. Thus there are no distinguishable terms between which the relation of ‘being the same as’ could hold (for, as we have seen, it is a relation, and requires two terms at least). In fact the idea of time does not strictly apply to this unchanging entity at all. Where there is time, there must be succession; and in this entity, ex hypothesi, there is none.’

Although there appears to be no change in the object, there are changes elsewhere. He used the example of a stone sitting amid a field of grass. When the wind pass by this field, the blades of the grass would move, or in other words change. However, the stone sitting amid the grass would appear to be unchanged. This leads us to a ‘fiction’ where the changed object is somehow amid changing objects. But this is nevertheless a fiction, because the stone although appears to be unchanged, it is still changing. The change the stone participate in is the change of time or of succession in time. To be clearer, the stone looks as if it is unchanged among a sea of waving grass, but it is indeed still changing in successions of moments. This is where we get the mistaken idea of identity. Quote:

‘We arrive at the notion of something which is at once multiple and unitary, by conceiving of the one object as existing at many ‘points of time’.‘

But how did we manage to confuse the idea of resemblance with the idea of identity? The answer is our imagination. The imagination upon sensing two very similar sense-perceptions, although different in time, immediately joins them together. And this is exactly the same as constancy. Our mind goes from A1 to A4 despite the moments of non-perception because it imagines the object to be the same and ‘the passage from one moment to another is scarce felt’. Our imagination is lead to fill up our interrupted sense-perceptions with additional ‘unsensed sensibilia’. Now, we would not have felt this at all if our imagination works perfectly – if we don’t feel any interruptions in our sense perceptions and always believe the object to remain the same. But that is not the case, because we do sense these gaps in our sense-perceptions. We do this by supposing there are unsensed sensibilias in-between those sense-perceptions. We then complete our interrupted sense-perceptions by filling it up with unsensed sensibilias to form one whole sense-perception just like the normal sense-perceptions we could have if our sense-perception is not interrupted. We did not just assume, we actually come to believe in those unsensed sensibilias. The unchanging characteristic of the stone from earlier is only an appearance; it only appears to be unchanged among the grass. The unchanged appearance of the stone is due to its relatively monotonous appearance through time, so its qualitative characteristics appear unchanged. However, the stone although appears to have not changed, it nevertheless still have temporal parts, because we could divide the perception of the stone into a set of relation of temporal parts. We also know from past experience that this seemingly unchanged perception could be interrupted by such things as a blink or the turning away of our head. Therefore the seemingly unchanging perception is actually a series of highly resembling perceptions that can be divided into temporal parts. This is as far as we have to go for our present purpose. Hume concluded that our belief in the existence of an external world is out of habit and our imagination. Hume’s proposal for the problem of the external world is to ignore it; which no one accepted.

The matter is more complicated with Kant, after reading Hume’s Enquiry, Kant decided to establish some kind of rule that can secure philosophy and the sciences from skepticism (it’s debatable how successful he was). It’s too complicated to explain in this article; therefore we’ll only look at Kant’s view on the external world. Kant accepted Hume’s view that all we get from the external world are sensations (or sense-perceptions). But he disagreed with Hume in other aspects. Kant explained that although things such as ‘the relationship between cause and effect’ and ‘space and time’ may not exist out there, we still sense them. And that is why they are important. The external world supplies us with raw data, which in turn are ordered by OUR mental apparatus into categories of causality, space and time, etc. For example, when we feel something as heavy, the property of heaviness does not belong to the object, it belongs to our sense-faculty (as a thing can appear both heavy and light to different person and it would be a contradiction to say an object contains both heaviness and lightness. Which is also the argument earlier philosophers including Descartes used against secondary qualities being actual qualities instead of things perceived by us). For Kant, the source of our perceptions is caused by something he called ‘thing-in-itself’, which meant something independent of all of our perceptions. It follows from this that if we can only perceive things in categories of ‘space and time’, ‘cause and effect’, etc, then there are things we cannot perceive at all (qualities aren’t the thing-in-itself, if we strip away all qualities of an object then all that remains is an unknown something as the cause of our perceptions. It also follows that we never directly perceive the cause of our sensations, but we only have a mediate perception through our mental apparatus. The external world of thing-in-itself is unperceivable for Kant (although he claimed we could somehow know it), which echoed Berkeley (if we take away his God as the cause of our sensations) and Hume’s theories. How useful is it to talk about things we can’t perceive? Many subsequent philosophers (Schopenhauer, Wittgenstein to name a few) asked this question and came up with various answers. However it’s still praise worthy for Kant to come up with this distinction.

In conclusion, it may be asked whether philosophical wonderings on the nature of the external world is worthwhile (as we’ve seen, everyone seem to have a different theory from everyone else). Many do not think so and look on such behaviors as a waste of time. However it must be pointed out that they are useful and worthwhile in that they provide us with better understandings of the world we live in. One of great scientists – Einstein had read all three of Kant’s critiques when he was 16. He went on to claim the cause of his theory of relativity is due to Hume and once remarked that he would never have dared to overthrow the science of Newton if he had not read Hume.