300 – Rationalism vs Empiricism – Summary and History

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300 – Rationalism vs Empiricism – Summary and History

What is reality really like? A current running through much of the philosophical thinking around the time of Socrates and Plato was that there is a difference between how the world appears and how it is. Our senses reveal one layer of reality but it is our minds that penetrate deeper. The world of appearances is a world in flux but underneath there must be a stable reality. For there is much that is unchanging. We recognise kinds of things – badgers, daffodils, mountains – and whilst members of these kinds are born, change and die, and differ from one another in ever so many ways, the kind-defining essence doesn’t change.

We see here the key rationalist idea that knowledge is a priori knowledge of necessary truths Plato said that kinds were defined by the transcendental forms. He presented a number of arguments for the existence of these things. Prior to our incarnation, our souls existed in the realm of forms where we learned about these essences. In our terrestrial state, we cannot recall what we know. Socrates considered himself a “midwife to knowledge” instead of a teacher, helping his interlocutors to draw out what they don’t know that they know.

The example of Meno and the slave-boy shows this idea clearly. Like many philosophers, Plato was also fascinated by mathematics. We are able to tap into a universe of truths that are non-sensible: we do not see numbers and we do not see the perfect geometric forms. Once again, we see the difference between the powers of the mind and the powers of the senses. It was in the 17th century that the debate between the rationalists and the empiricists came to a head. Philosophers such as Descartes and Leibniz emphasised the power of reason over the senses.

Descartes argued that our senses were fallible and that we could not rule out the possibility of the demon deception hypothesis on the basis of sensory evidence alone. Descartes argued that he knew he existed, as a mind, on the basis of reflection alone: when I think, I cannot fail to be aware of myself as existing as that thinker (cogito, ergo sum). Having proved that he exists, Descartes argued that God exists. Since God is no deceiver, he would not have given us senses that systematically mislead. But let us not overemphasise the powers of the senses.

Descartes argued that even with material things, it is reason that exposes their essences. In his piece of wax reasoning, he argued that the senses merely reveal a succession of impressions: it is reason that grasps the underlying and enduring substance as extended (and filled space). Plato and Descartes believed that we are born with concepts and knowledge. In Descartes’ case, there was a religious motive: we are all born in the image of God. We discover more about the world primarily through metaphysical reflection. The philosopher Francis Bacon, an early empiricist, famously dismissed this rationalist approach to knowledge.

He compared rationalists to spiders who spin “complex metaphysical systems out of their entrails”. Empiricists get their hands dirty: like bees gathering pollen, they gather knowledge about the world and only then reflect on it. Around the same time as Bacon, many new discoveries were being made that shook the prevailing views of reality. The Earth was dethroned from its position at the centre of the universe by Copernicus. A new star (a supernova) was observed by Tycho Brahe in 1572 – yet the heavens were supposed to be timeless and unchanging.

Galileo discovered the moons of Jupiter – again, everything clearly didn’t revolve around the Earth. Later in the 17th century, scientist-philosophers such as Newton, Boyle, Gassendi and Huygens would revolutionise our understanding of reality. The original empiricist manifesto was written by John Locke. In his Essay Concerning Human Understanding, he sought to show how a mind that was blank at birth – a tabula rasa or blank slate – could come to be filled. His first targets were the innate concepts and knowledge (‘ideas’) of the rationalists.

There are no such things. There are no truths everyone agrees on. Many people fail to grasp the supposed metaphysical truths. Instead, our senses deliver ideas to us. We store them, abstract from them to form general ideas, and compound and mix them to generate new ideas. Like Lego bricks, we build the meagre sensory data into ever more complex structures. Even Leibniz thought Locke was onto something here. He claimed that our minds were like blocks of marble that had to be carefully chiselled at to reveal the hidden structure (the innate truths).

It is hard work and not everyone will end up well-chiselled. Hume took empiricism to its limit. Where Locke talked indifferently of ideas, Hume distinguished impressions and ideas. Impressions are the direct deliverances of the senses and are forceful and vivid in comparison to ideas, which are the copies our minds makes. (He also agreed with the Empiricist Berkeley that Locke’s theory of general ideas was wrong. We do not abstract from particular ideas to a general idea but use a particular idea in a general way via a general name. )

What about the precious necessary truths philosophy is supposed to study? Locke argued that once we have ideas in our mind, our mind will perceive the necessary connections between them – e. g. that a triangle has internal angles that add to 180o? But where does the idea of necessity come from? Hume provided an answer. He distinguished statements into two categories: those expressing relations of ideas (analytic) and those expressing matters of fact (synthetic). The analytic truths express mere definitions: we simply are aware of an association between terms.

The synthetic truths are the contingent truths. So what happens to interesting necessary truths, such as God exists or nothing exists without being caused to exist? Hume argued that if these weren’t analytic – and they aren’t – they aren’t necessary. We feel that they are necessary and this is all necessity is: a psychological property. When we say that X caused Y, we think we have said something about the universe. We think we have seen an example of a law of nature (e. g. the water in the bucket froze because it was cold exemplifies the law water freezes at 0oC).

Science investigates these laws. Hume said that causation was “all in the mind”. We see one thing after another and when we’ve seen instances of a regularity enough, we develop the feeling that one thing must be followed by the other. Hume, like Locke, emphasised how all we can be certain of are our impressions – how the world seems. Scientists are really investigating how the world appears: they can never be certain that the world really is the way it appears. So, empiricism seems to lead straight to scepticism about the external world. Kant objected strongly to this.

Science really is studying the external world and there really is an external world for it to investigate. Kant brought about a revolution in philosophy (he called it a “Copernican revolution). He argued that the empiricists and rationalists were both right and wrong. The Empiricists were right: science requires the study of the world and the world is brought to us via the senses. The Rationalists were right: our mind is not blank but contains structures that enable us to interpret the stream of data from the senses. We may liken the mind to a mould and the data to jelly: one only has something structured by combining both.

Or: the mind is a computer with an operating system and the data is the input from the user. A computer with just an operating system is inert. A computer into which data is inputted but which has no operating system is just data: it cannot be interpreted. Only when you combine both do you get something useful. Our minds contain the “structures” for space, time, objects and causation, for example. (In Kant’s terminology, space and time are the pure forms of intuition whereas the structures for objects and causation are pure concepts of the understanding.

) This means that we experience a world of spatio-temporally located objects in which causation happens because this is how our minds make it appear. Does this mean that the world as such is “all in the mind”? Or is the mind somehow “tuned” to the structure of reality, so that our pre-programmed minds mirror the structures of reality? This is a very difficult question over which there is no agreement amongst experts. The Empiricist movement came back with a vengeance in the 20th century. Philosophers such as Bertrand Russell agreed with Hume that our knowledge begins with our knowledge of sense-data (classical empirical foundationalism).

Armed with new discoveries in mathematics and logic, and backed by the successes of science, the logical positivists argued that the only proper way to investigate the world was the scientific way. If I say p and p is synthetic and there is no objective, scientific way to verify my claim that p, then my claim is meaningless. (This is the celebrated verification principle). So, if it is true that there atoms, we should be able to find empirical – sensory – evidence of them. If it is true that nothing happens without being caused to happen, then we likewise need scientific evidence for this.

We cannot discover whether it is true by pure reason. The Logical Positivist movement failed. There is much that seems meaningful that is not objectively verifiable by the senses, such as the occurrence of private sensations. The principle makes it impossible for general claims such as “all mammals are warm-blooded” to be true, as we cannot verify all of them. The very verification principle itself fails its own test! The Logical Positivists responded by watering down their principle: a meaningful claim is one we could gather some evidence for in principle and the principle itself is special – exempt from this rule.

But it was not enough. (* Then Quine argued that the fundamental division between analytic and synthetic sentences was incorrect. Analytic sentences cannot be false. But no sentence enjoys this privilege. As we learn more and more, truths we thought were beyond doubt are rejected. Once upon a time, we would have thought it analytic that no object can be in two places at once or that there is no fastest velocity. Quantum physics and general relativity theory show that they are not true. Instead, we should have a “web of belief”. At the centre are those sentences least likely to be revised – our “core beliefs”.

As we move out, we find those sentences that would be easier and easier to accept as false – that would cause less and less disruption to the rest of what we believe. ) In the 1950s, Chomsky became famous for suggesting that we are not born as blank slates when it comes to language. We are born knowing the fundamental structures of human language. When we are young, we hear our mother tongue and use our knowledge of language to pick up our language very quickly. (At 24 months, the average child understands 500-700 words; at 36 months, 1000; at 48 around 2500-3000; at 60 around 5000 words: that’s around 7 words a day between 3 and 6).

More recently, studies have shown that children are born with brains structured to “expect” the world to behave in certain way. Very young children expect objects to persist over time: not to disappear and reappear at two different places, for example. Is this a revival of rationalism? Not according to many people. Rationalists argued that we had innate concepts and knowledge. By reflection, we can discover them and manipulate them to gain new knowledge. But our “knowledge of language” is altogether different. None of us can easily articulate the rules we follow in generating syntactically-correct English.

(And certainly none of us at all can articulate the “common structure rules” to all human languages. ) Our brains are certainly pre-programmed, but only perhaps in the same way that a computer is pre-wired: clearly something has to be there but nothing as advanced as software. So where are we today? No side is “victorious”: this would be to grossly over-simplify the debate between the empiricists and the rationalists. We definitely have minds in some way “ready” to receive the world – hardly surprising, perhaps, given the time it has taken for us to evolve.

But when it comes to working out what is true? Few philosophers are rationalists in the old-fashioned way. There is no sharp division between metaphysics and science: our study of reality cannot be done from the armchair alone. But our capacity to grasp abstract mathematical truths has always been difficult to explain from an empiricist perspective. We seem to have an access to a mathematical realm and a cognitive or intuitive access instead of a sensory one. You can’t see numbers, after all, and it is not easy to say what we could “see” that would lead us to generate the ideas of numbers.


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  • University/College: University of Chicago

  • Type of paper: Thesis/Dissertation Chapter

  • Date: 3 November 2016

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