Introduction: doubt
You've probably heard of Rene Descartes' idea that cogito ergo sum; or, "I think therefore I am". Now, it has amused wise-asses for centuries to point out that this contains a logical fallacy, and that in fact all that is proven by the fact I'm thinking is that thinking exists, not me. This may be an important point, but it does not concern me here. Every great thinker's work contains contradictions and fallacies; this is why they continue to think. What I want to address here are the ramifications for the modern world of Descartes' method, de omnibus dubitandum est; or, "everything is to be doubted".
The writings of philosophers have never constituted a decisive historical event, and clearly the modern world is not as it is because of what issued from Descartes' pen. But it is indisputable that doubt of everything has become the cornerstone of modern thought and experience, and that this has been reflected in the subsequent development of modern philosophy, politics, and science. Doubt of everything - of data from the senses, of religious revelation, of rational thought - is the hallmark of the world we live in.
Doubt of the experience of the senses dates back primarily to the discovery of the fact that the Earth revolves around the Sun and not the other way around, which is contrary to our experience of watching the Sun make its merry way across the sky every day. Indeed, the more sophisticated we become at measuring and observing reality through our instruments, the more alienated from the world we are: I know that this desk is made up of trillions of tiny little particles and so is my hand, but if I poke it then my felt experience is rather different. Clearly I cannot trust my senses to reveal reality to me if this is the case.
Doubt of rational thought is rather different. Ancient philosophers used to think that reason was a way of discerning the truth, so that by thinking about things I might be able to arrive at truth somehow. But how, Descartes said, could I be sure that my reason wasn't being deceived by an evil spirit? My reason isn't necessarily a tool which can be used to understand reality because I haven't got any proof that what I think I know about reality is the case - especially with this pesky problem concerning my senses. The only reason that Descartes felt he could be sure of was logical or mathematical reasoning of the kind that says that 2 + 2 = 4. But this isn't a truth about the outside world, only one about the human mind: this sort of logic makes sense in my head, but it doesn't signify anything about the reality of the outside world.
Doubt of religious revelation is, of course, the simplest form of doubt to understand; doubt is inherent in the fact it is faith, and not reason, which we ultimately have to resort to if we choose to seek God. The existence or non-existence of God cannot be proven through sense experience or rational thought and there has been doubt of revelation for as long as there has been revelation. And in fact Descartes had to eventually invoke God - whose existence he didn't doubt - to save the existence of reality. But for most of atheist modernity, this isn't an option.
Modern politics
The prevalence of doubt has meant a retreat in the realm in which man can be said to perceive the truth. This turns on its head the tradition of Judaism and Christianity, as well as secular philosophy; all were based on the idea that the truth about the nature of the universe and man was out there somewhere beyond our own minds, and that we could access it through religious revelation or, in Platonic philosophy, contemplation of higher truths which he called "forms". The concomitant retreat of claims to political and social knowledge can be seen in our political systems, now founded theoretically on Hobbes' simple injunction not to harm anyone, and we even tolerate intolerance because we no longer believe in our right to suppress it.1
What is peculiar about modern philosophy is the extent to which it is based on introspection, on the experiences that an individual has within their own consciousness. I remember the first time I picked up Hobbes' Leviathan, expecting to dive straight into a discussion of his political philosophy; instead I discovered over one hundred pages 'On Man', and that was quite enough to discourage me for some years. But what I failed to realize during that first abortive reading was that Hobbes, and indeed most modern political philosophy, had been thrown back on a discussion of man by the Cartesian doubt. If all I can know is myself - because there's no revelation, sense data, or rational truth which I can trust - then I have nowhere else to look to investigate the truth about anything except within myself, including the truth about how to organize political life.
I have written about Hobbes elsewhere, but it is worth briefly recapping, because he was the most uncompromising advocate of ideas which have appeared again and again throughout the modern period.
Hobbes was a materialist, which meant he didn't believe in anything apart from matter - that meant no soul, no Platonic forms, and no immaterial God (he still believed in God, but his refusal to believe in the immaterial got him in all sorts of trouble which need not concern us here). And because he didn't believe in the soul or even in the mind as something immaterial, all he was really sure of was the passions. Passions - like my desire to eat or to learn - are things that emanate from within me without any stimulus, and drive me to go and make a sandwich or read a book: so I can be as sure they exist as I can my thought.
"Descartes proceeded by thinking all things out of existence and saw that when he tried to think thinking itself out of existence, he was still thinking."2 Well, to put it in simple terms, Hobbes proceeded by thinking all things out of existence, and saw that he still wanted a sandwich. So he was sure that man had passions - desires - and that the strongest of these desires was the fear of death, which became the cornerstone of his philosophy. Everyone has the right to avoid violent death, and so they form a commonwealth where they all agree to give up the right to kill each other. The rule governing this commonwealth is "Do not unto others as you would not have done unto you", and it doesn't involve any other obligations beyond keeping one's agreements. After this commonwealth has been established, everyone goes about his merry business of acquiring property and following his passions.
This idea was founded by Hobbes entirely on an investigation into his own being. How alien this was to the history of western thought - from Plato's belief that politics should exist to make the quest for truth possible, to the Christian belief that politics served the ends of God - is shown in Hobbes' claim that the history of political philosophy "is no older than my own book De Cive". Prior philosophical enquiries into politics had considered much more than simply man's passions. And Hobbes is rightly seen - along with Machiavelli - in taking the study of politics in an entirely new direction. His ideas are essentially those which lay behind the theory of the small state which allows capitalism and private interests to run its course; and they came about because the Cartesian doubt eliminated everything but our own selfish passions, the existence of which we can be sure.
Morality
All this has rather strong implications for traditional notions of morality, be they religious or philosophical. If morality consists entirely of keeping my agreements with others and not killing anyone, then most of the tradition of moral thought needs to go out the window. If the Cartesian doubt has eliminated the possibility of me deriving morality from God or from natural law, then I really have nothing else on which to base it but my own selfish passions and the legitimate claim of the passions of others.
Of course, this did not happen: but this was merely a matter of inertia. Across western countries, traditions of morality continue. But what has changed is the loss of a theoretical basis for them - when challenged, they have no firm ground on which to hold off the assault. Asked why murder is wrong, people will respond "it just is"; well, I agree, but this truth-claim is no more persuasive than that of a terrorist who claims otherwise. What the Cartesian doubt has done is deny us moral truths and left us with "values", which are interchangable as no one value is inherently any truer than another. It's inherent in the word: the "value" someone places on something is clearly relative to the person doing the placing.
The declining applicability of the idea of "truth" to modern political and moral life is, I would suggest, the reason for the explosion of the "culture wars" in America in recent decades. Abortion is a case in point: as soon as it is no longer an accepted truth that each fetus bears the mark of a divine Creator and hence that abortion is wrong, the issue passes into the realm of values and opinions. And so on with the proliferation of all sorts of values and opinions which we no longer have any criteria for judging beyond our own prejudices, and whether the opinions affect public order. An individual might still believe in revealed or discovered truth, but the political system does not; and so his belief is no more valid than that of anyone else with an opinion.
So modern morality has left us in this situation where it is sheer life itself that we revel in and protect, and has really reduced the main thrust of our politics to the running of an economy which makes the experience of living as comfortable and easy as possible. And while life is comfortable and easy, little questioning of our notions of morality takes place. But what can happen when this breaks down has been seen in Nazi Germany, where the collapse of the Weimer Republic revealed that if a people under huge stress plunge freefall into the abyss of value relativism then what comes out the other side is not necessarily a shiny happy democracy.3 If there is no truth and all values are relative, then Nazism is as legitimate a response to this as anything else. In other words, if liberalism and Judeo-Christian morality are not truths, then their survival are an open question.
Conclusion
So the Cartesian doubt has led to value relativism, and the problems I have described. At this point, it is inevitable to invoke Friedrich Nietzsche, who pointed a way out of this trap. Nietzsche has been called a "life philosopher" because he gloried in man's creative capacity - again, after being thrown back on introspection by the ramifications of the Cartesian doubt. He admired men who had freely created systems of values through their wills. He once wrote in his notebook, "In that which moved Zarathrusta, Moses, Mohammed, Jesus, Plato, Brutus, Spinoza, Mirabeau - I live, too". And by this he meant that he had the courage to create new values by sheer force of will.
Men like Abraham, Buddha and Jesus created systems of value which were so strong and capable of reproducing themselves that they gave human meaning to the world. "It is not the truth of their thought that distinguished them, but its capacity to generate culture."4 These people were the founders of whole cultures which were based on the systems of values that they created - and it is this process that Nietzsche would like us to repeat, or thinks we will be forced to after too long tumbling into the abyss.
Until such a time as our political and social life undergoes such stress that we will be forced to come up with new values, utility is our God: sheer usefulness for comfortable living is the measure of most things, and peaceful economic growth the goal of politics. The Cartesian doubt and value relitavism are, in a way, luxuries we can engage in while life is easy; it is no coincidence that they have taken hold in the most prosperous societies mankind has ever known. But if we should ever go through an experience such as did the Weimar Republic - the failure of economic progress and massive civil violence - then we will find that the Cartesian doubt has destroyed many of the bases on which we might have crafted a humane and civilized response. What happens next will be truly unexpected.
Notes
1. There are of course laws against Nazism in Germany and Austria, and against advocating violence against religious or racial groups in other countries. But these are based on the need to preserve public security, not a general intolerance of intolerance. That these laws have emerged only after extreme violence in the former case and the reality of Islamist terrorism in the latter is proof of this.
2. Heinrich Blucher, Lecture on the Common Course: http://www.bard.edu/bluecher/lectures/com_crse_talk/talk_page4.htm
3. This truth is hidden by the widespread assumption that Nazi Germany was the result of an ominous extension of power by a stable state, and so hence this experience might be repeated in a place such as the United States by extending powers of surveillance, arrest, etc. In fact, the Nazi movement's triumph was not the result of a state which slowly acquired too many powers of internal security, but the utter breakdown of internal security; the United States would have to undergo total economic catastrophe and virtual civil war to recreate the experience of the Weimer period.
4. Allan Bloom, The Closing of the American Mind (Simon and Schuster ed. with a foreword by Saul Bellow), p. 201
Further reading
Four books I have read moulded this. Most importantly, Hannah Arendt, The Human Condition, especially the sixth section. Then Leo Strauss, Natural Right and History; the book by his student Allan Bloom, The Closing of the American Mind; and finally, because it is slightly off topic, Arendt's The Origins of Totalitarianism.