Latorshia asked:

Why doesn’t Descartes simply determine what’s real by looking around him and use his sense experience?

Answer by Helier Robinson

Your assumption is that everything empirical (i.e. known through the senses) is real; but this is not so, because some empirical data are illusory and thereby unreal. Illusions are contradictions within sense experience (e.g. the half immersed spoon in a glass of water, which is bent to the sight and straight to the touch) or between sense experience and well established belief (e.g. the railway lines appearing to meet in the distance) and there are no contradictions in reality. The business of finding out what in the empirical world is real is empirical science (as opposed to theoretical science, which tries to explain empirical reality by describing its underlying causes) and empirical science is still not yet complete.

Empirical reality may be defined as everything empirical that is not illusory, as opposed to theoretical reality which is every that exists independently of being perceived. It is widely believed that these two definitions are equivalent but in my opinion they are not, because of the argument from illusion. This is the argument that since illusions are unreal they are misrepresentations of reality rather than reality itself. In other words, illusions are false images of reality. But there is no clear distinction, in perception, between illusions and non-illusions, so non-illusions must be images of reality also. That is, illusions and non-illusions are all made of the same ‘stuff’, namely, sense data, which are sensations provided by the sense organs. The argument from illusion is widely disregarded because its conclusion is so opposed to common sense, but it has never been satisfactorily refuted.


Answer by Craig Skinner

Because he thinks REASON comes before OBSERVATION as the route to secure knowledge. He is a rationalist, as opposed to empiricist, philosopher. Of course all agree that reason and observation both contribute, the distinction between rationalists and empiricists is only rough and ready. and one of emphasis.

As a rationalist Descartes must reason his way to the view that he can trust his senses, not start off by taking sense experience as a reliable basis for knowledge. He notes that sense experience suggests, for example, a flat Earth, unmoving Earth, small sun and tiny stars – all incorrect.

Descartes is disenchanted with medieval philosophy based on Aristotle (who was pretty much an empiricist), and says he wishes to start from the beginning and build a solid foundation of knowledge for the sciences. His method is the famous Method of Doubt or methodological scepticism: he will only accept as knowledge what is so clear and distinct to his mind that it can’t be doubted, proceeding from there by reason.

He begins by doubting the evidence of his senses including the existence of the external world and his own body (at any time, he might be dreaming, or an evil demon could be deceiving him) Then he doubts even truths of reason (an evil demon could affect his mind tricking him into thinking 2+2=4 when it’s really nothing of the kind).

After all this doubting, is there anything left he can rely on? Yes, the fact that he is doubting means he is thinking, so he must exist (I think therefore I am, ‘cogito ergo sum’ in Latin).

Can he move on from there ? Is there anything else which is clear and distinct ? I suppose most of us would go straight from the Cogito to saying we had a clear and distinct idea of an external world including our own bodies, gained through sense experience. But, for Descartes, such a jump is unjustified. He says he has a clear and distinct idea of God as an omnipotent, omniscient, infinite, necessary being, and then argues as follows:

I. God therefore exists

2. God is no deceiver, so

3. I can (mostly) trust my God-given senses and reason.

4. My senses yield a clear and distinct idea of an external world including my own body.

5. These exist.

This argument from the Cogito back to the everyday world via God as a bridge is full of holes – do we all have this alleged clear and distinct idea of God (I don’t); even if we do, this doesn’t mean there is any God answering to this idea: even if there were a God, why shouldn’t he be a deceiver?

So he is no nearer to establishing the existence of the external world with CERTAINTY than he was when he started. But, as he says himself in the Synopsis of the Meditations, ‘no sane person has ever seriously doubted these things’. And of course it’s well known to the beginner in philosophy that the existence of the external world can’t be proved – solipsism, idealism, brains in vats, world-including-all-of-us as a simulation, all logically possible. But nobody really believes these things. One or two current philosophers argue that since there could be many simulations run by advanced intelligences but only one real world, it’s statistically more likely that we are virtual beings in an elaborate simulation rather than real flesh and blood humans in the real world, but I doubt they really believe it. If you are unsure, make sure you lead an entertaining (virtual) life so the higher intelligences stay interested and don’t switch off the simulation.

A final. important, philosophical point. Can we trust reason more than our senses ? The rationalist traditionally argues that reason yields a priori, analytic, necessary truths; observation yields second-class, a posteriori, synthetic, contingent truths. I wont go into the modern debate as to whether the analytic/synthetic distinction exists or whether the a priori is a coherent notion. But why should we trust our reason more than our senses?. Both can go wrong. Neither can be justified in any absolute sense – our senses can deceive us sometimes, and, logically speaking, could deceive us always. Likewise with our reason – we can only justify reason by reason, a circular argument. Ultimately we just accept our senses and our reason as generally reliable guides to the world. We can say, with Descartes, that they are God-given and so reliable, or, post-Darwin, that they are part of evolved human nature, promoting survival by being reliable guides to the world. My general view is that the senses and reason are on a par. As the Scottish philosopher, Thomas Reid, observed in 1764:

‘Why, sir, should I believe the faculty of reason more than that of perception? – they come both out of the same shop, and were made by the same artist: if he puts one piece of false ware into my hands, what should hinder him from putting another?’

So, in the end, Descartes does determine what’s real by looking around him, like everybody else, but with a deeper understanding than he had when he started, and we all do well if we manage the same.