Does all knowledge come from experience or is knowledge, properly so called, gained by pure reasoning? Discuss with some reference to the philosophers of the 17th and 18th centuries.

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The two main philosophical traditions of the seventeenth and eighteenth century were Rationalism, led by Rene Descartes and followed by Spinoza and Leibniz, and the Empiricism of John Locke, Berkeley and Hume. The groups proposed very different answers to the question of how to go about gaining knowledge. The Rationalists proposed that true knowledge could only come through reason, the Empiricists said that true knowledge came from sensible experience. Rationalists argued that knowledge gained through the senses could not be trusted and could never reveal real truth. Empiricists objected that without reference to the real world knowledge would be meaningless. In this essay I will examine the ideas of Descartes and Locke to illustrate the opposing view points.

1 Descartes

Descartes was primarily motivated by a search for certainty, he wrote:

I had been nourished by the humanities since childhood ... [but] as soon as I had finished my course of study ... I found myself embarrassed by so many doubts and errors, that it seemed to me the only profit I had had from my efforts to acquire knowledge was the progressive discovery of my own ignorance... (Discourse on Method)

...I was struck by the large number of falsehoods that I had accepted as true in my childhood, and by the highly doubtful nature of the whole edifice that I had subsequently based on them. I realized that it was necessary ... to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last. (Meditations on First Philosophy)

1.1 The Method of Doubt and 'Cogito Ergo Sum'

Descartes wanted to rebuild the edifice of human knowledge on certain foundations, he began by asking, what can be doubted? Unless he could be absolutely certain of something he would refuse to accept it as a basis for true knowledge.

Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once. (Meditations on First Philosophy)

He could doubt his senses because they were sometimes deceived, by illusions or trickery. Dreams were experiences that seemed real, but were not. He doubted his reason because he sometimes made mistakes in reasoning. As an antidote to his habitual opinions he supposed that there was some powerful demon who could deceive him completely, so that if there was the slightest doubt about anything he would have to treat that thing as completely false. After all this he found that the only thing he could not doubt was that there was someone to do the doubting. He had to be there to be deceived. Whatever happened he was still there, the demon could not make him doubt that. This is the famous phrase 'Cogito Ergo Sum' or 'I think, therefore I am'.

The next question was what is actually referred to by 'I'? To answer this question Descartes proposed another round of rigorous doubt. He decided that his demon was capable of deceiving him into believing that he had even a body, any process related to his body was therefore subject to doubt. Finally he hit upon the answer:

Thinking? At last I have discovered it - thought; this alone is inseparable from me. I am, I exist - that is certain. But for how long? For as long as I am thinking ... At the present I am not admitting anything except what is necessarily true. I am, then, in the strict sense only a thing that thinks... (Meditations on First Philosophy)

Descartes concludes that he is an essentially thinking thing, and he could potentially exist without a body. It is at this point where most people find a major snag, if our minds are not a part of the extended universe, ie. have no physical properties, how do they affect the physical universe? How can anything that happens in our minds have any influence on the world? Many agree that this question has never been answered satisfactorily and, with our modern understanding of the physical universe, is unnecessary. Spinoza proposed that physical things and mental things were just aspects (or modes) of the substance of the universe. Thus they were in fact different aspects of the same thing, and a complete explanation of the physical aspects of the mind could not include any details on the mental aspects and vice versa. This answer hasn't really been improved upon.

1.2 Clear and Distinct Perception of Fundamental Knowledge

Descartes examines his perceptions of a piece of wax. He looks at the wax and describes it using all his senses - how it smells, how it feels etc. This, most people would accept, would be a complete description of the wax. He then leaves the wax by the fire and it melts, now it has none of the properties previously ascribed to it, it does not match the physical description, but it is undeniably the same piece of wax. There must be some aspect of the wax that is unchanged, some deeper understanding we have of what constitutes wax. Descartes concludes that there is some essential essence to the wax which is not perceived with the senses, so it must be perceived by reason.

Descartes argued that true knowledge could not come from the senses but only from the application of reason. This leaves the problem of what to reason from, reasoning only produces fresh knowledge when there is knowledge there to work with. He said that he could perceive, clearly and distinctly, in his brain certain things, these things were self evident and could be the basis of all thought. The ability to discern truth is fundamental to Descartes' argument, we must be able to discern within us those things that are true and he spends some time on the point. Of course, the problem is that anyone can look inside themselves and perceive things clearly and distinctly, and those things would not have to be the same things that Descartes perceives clearly and distinctly. The only argument on offer is that if we disagree with what is perceived clearly and distinctly, we are not actually perceiving clearly and distinctly. This is not an argument likely to convince the cynics.

2 Locke

2.1 Attack on Innate Ideas

Locke disagrees that we are born with any true knowledge, we do not have innate ideas of any kind - we are only born with the possibility of having ideas. This view has been somewhat discredited in the twentieth century through the work of the linguist Noam Chomsky who has shown that we are born with an innate idea of the structure of grammar. We seem to be born with the knowledge that there is language and words, and that those words are divided into certain basic types. Language acquisition in infants would be impossible without these preconceptions (and indeed doesn't happen when certain areas of the brain are damaged at birth). However, it is not clear that this contradicts with Locke's position, he certainly denies the possibility that we are equipped, at birth, with awareness of logical axioms such as "That it is impossible for the same thing to be and not to be." But, when he claims that the mind is filled with ideas through the senses he overlooks the difficulty of obtaining 'sensible' information. Processing the visual images from the eyes requires a lot of work, we certainly are never conscious of learning how to see. It is clear that the newborn child has to have some innate abilities, or preformed structures for accepting sense data, but Locke claims that these do not amount to ideas.

Though the qualities that affect our senses are, in the things themselves, so united and blended, that there is no separation between them; yet it is plain, the ideas they produce in the mind enter by the senses simple and unmixed ... the hand feels softness and warmth in the same piece of wax; yet the simple ideas ... are as perfectly distinct as those that come in by different senses. (Essay Concerning Human Understanding)

Locke scoffs at the theory of innate ideas, claiming that if we are allowed innate ideas then everything which we can conclude, through reason, from these ideas must also be allowed innate - there being no difference between these ideas and those derived from them. If this is the case then reason, which is,"nothing else but the faculty of deducing unknown truths from principles or propositions, that are already known," and,"We may as well think the use of reason necessary to make our eyes discover visible objects, as that there should be need of reason, or the exercise thereof, to make the understanding see what is originally engraven on it."

Locke allows that reason and intuition can provide knowledge:

Sometimes the mind perceives the agreement or disagreement of two ideas immediately by themselves ... And this, I think, we may call intuitive knowledge. For in this the mind is at no pains of proving or examining, but perceives the truth, as the eye doth light, only by being directed towards it. (Essay Concerning Human Understanding)

When we cannot immediately perceive the connection between two ideas we may, at a later time, hit upon a particular succession of ideas which show the agreement or disagreement of two ideas, this we call reasoning. This may seem like surrender to rationalism but the key issue here is the source of ideas. We may intuitively perceive the relation between two ideas but those two ideas will have arrived in the mind through the senses. There is, of course, the matter of the reliability of sense data brought up by Descartes but Locke is disdainful:

I believe he will allow a very manifest difference between dreaming of being in a fire, and being actually in it. (Essay Concerning Human Understanding)

He also argues, slightly more seriously, that if life were a dream then reason and argument are no use, and truth and knowledge meaningless.

2.2 The Origin of Ideas, Filling the 'Tabula Rasa'

Ideas, Locke claims, arrive in the mind only through experience, either through the external observation of sensible objects (sensation) or the internal observation of one's mind (reflection). In order to explain how these two sources can account for all knowledge Locke makes the distinction between 'simple' and 'complex' ideas. Simple ideas are things such as colours, textures, smells and tastes which the mind has,"the power to repeat, compare and unite even to an almost infinite variety, and so can make pleasure at new complex ideas." So our complex ideas are made up of combinations of simple ideas and, even, other complex ideas. If we imagine a yellow cow we have united to ideas, yellow - which we could perceive in a banana, and a cow, which could be considered a complex idea. We are incapable of imagining anything which does not have some source in our sensible experience. As I have already remarked this explanation ignores the difficulty of discerning ideas in sense data, something which has been a major stumbling block for computer vision systems. When we look upon a scene and see a collection of individual objects this is very different from the disparate patches of light and dark actually falling on the retina. There is also the problem of how ideas in the mind might be connected to things which exist in the world, a problem exploited by Hume to arrive at the conclusion that the physical universe did not exist unless someone perceived it. So a table should you cease to observe it would cease to exist. This somewhat absurd idea seems to be the ultimate consequence of Empiricist thought which some have taken to be a good reason for discounting to whole thing.

3 Conclusions

Descartes and Locke have many things in common, both attempt to answer the question of what can be known by asking, what can I know? What is it possible for an individual to know? They tackle this common problem from completely different angles. Descartes, a mathematician, searches for certainty and proceeds from there. Locke, the man of science, wishes to progress through different degrees of truth to approach certainty - eliminating doubt with each new discovery. It is doubtful that either present a full picture. Experiential knowledge without the application of reason, as even Locke admitted, will not progress far. Rational knowledge can quickly run into inconsistencies and pointlessness when it is not corroborated with knowledge from the senses. Both have been extremely influential, despite our advances in science over the past centuries it is clear we can still learn a great deal from both of them.


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Descartes, John Cottingham, Phoenix, 1997.

An Introduction to Philosophical Analysis, John Hospers, Routledge, 1997.

Philosophy 1 : A Guide Through the Subject, ed. A C Grayling, Oxford, 1998.

The Language Instinct, Steven Pinker, Penguin, 1994.

Classics of Philosophy Volume II: Modern and Contemporary, ed. Louis P Pojman, Oxford, 1998.

© Robert Crowther