"The most widely shared thing in the world is good sense, for everyone thinks he is so well provided with it that even those who are the most difficult to satisfy in everything else do not usually desire to have more good sense than they have.…" (Descartes, Discourse on Method)
Most of us are familiar with Descartes' famous idea that good sense is the best shared thing among the world. This seemingly innocent and well-meant idea has seen many critique, how could one say such a blatantly wrong thing? Isn't it obvious that not everyone's judgement is as accurate, that maybe, most people are stupid? And that good sense is only found in the few?
In a footnote in his edition of Descartes collected works, Ferdinand Alquié reminds us that Descartes wasn't as naïve as we might like to believe, he sure as hell knew that good sense wasn't the best shared thing in the world. Experience clearly says otherwise. But then why did he make this idea such a central point of his philosophy? According to Alquié, the argument should be compared to Kant's argumentation concerning the Ideas of Reason. For Kant, the three principal Ideas God, World, and Soul don't exist de facto, they are not to be found in experience, but they do exist de jure, they are necessary presuppositions for the orderly usage of reason. In a similar way, Descartes didn't believe that everyone is de facto endowed with good sense/judgement, but that this has to be presupposed de jure. For Kant, the presuppositions of the Ideas were needed for the orderly usage of reason.
For what end did Descartes need the presupposition of a good sense that is shared? Perhaps this has to be clarified by looking at the general goal of Descartes' philosophy. Descartes' wanted to escape from the presuppositions carried over from antiquity throughout scholasticism. The way of doing philosophy in the Middle ages was characterized by a deep and thorough engagement with the texts of predecessors, one developed one's own thinking through commenting on the established authorities. Descartes saw that, by doing philosophy in this way, it was impossible to escape from presuppositions. For example, if even the slightest error in thought was presupposed by Aristotle, and philosophy is done by commenting on Aristotle, this error could never be overcome. Extend this process through centuries and errors become so engrained that it eventually becomes impossible to overcome them.
What Descartes wanted was a sure basis from which thought could deploy itself without presuppositions. To do so, he had to start, not from other people's thinking (philosophical authorities), but from thought itself. Hence the famous lines that he had grown so tired of wandering across the world (the different texts of other philosophers) that he eventually had to return home (his own reason). To do so he had to discard all the ideas he had adopted from others, and start anew. But how can one do this? One has to first believe that one is able to do so, one has to presuppose that one's own reason is as good equipped as that of Aristotle's or Aquinas's to do the work that is required by philosophy.
"…I entirely abandoned the study of letters, and resolved no longer to seek any other science than the knowledge of myself, or of the great book of the world.…" (Descartes, Discourse on Method)
You see, the entire method of doing philosophy in the Middle ages was based on the idea that one could only think by first absorbing the accepted truths of the tradition, one had to presuppose that one's own reason, left to itself, was unable to grasp the truth. And it is this conception of doing philosophy that Descartes was reacting against, and that is the basis of the idea that good sense is the best shared thing in the world. His statement was never meant to be a statement of fact, but was meant to express what was necessary for us to believe, in order to bring about a new way of doing philosophy. His engagement wasn't about true knowledge, it was social and political in nature.
In order to do philosophy in a way that broke with the presuppositions carried over through tradition, it was first necessary to believe that our own thinking is as powerful as that of our masters. Many have critiqued Descartes on this point, often missing the point of his statement. When Spinoza claimed that men are born idiots, and that it is only through education and a violent imposition of culture that they become knowledgeable, I suppose that Descartes wouldn't disagree. He would merely laugh at the self-evident and banal nature of Spinoza's claim. It is evident that we are born idiots, lacking knowledge, but this is not what matters.
Let us look at what can happen when we choose to believe the contrary of Descartes' claim, to see why his counter-intuitive claim is of importance. What happens if we suppose that the capacity for sound judgement differs in men, and that only a few are capable of it? What happens is that man stops believing in himself, and presupposes that he first has to be educated before he can speak his truth (as Spinoza claims). Of course, there are always men more educated than oneself, and we soon realize that there is way too much to learn, so that we should better focus on one specific branch of reality in which we can gain knowledge. So that, maybe, we'll at least be knowledgeable about one topic. This results in hyper-specialization and the habit to depend on other people when it comes to domains that we aren't highly educated in. We all know where this leads. We are discouraged to do our own research, and listen to experts instead.
Are we in this sense better off than the Middle ages? I don't think so. The critiques against Descartes' phrase, from Spinoza through to the current enforcers of a 'reign of specialists', are encouraging the thing that Descartes was reacting against. A climate in which one is discouraged to use one's own reason and is left at the mercy of others. The idea of a shared good sense as posited by Descartes is essential to the emancipatory character of enlightenment thought. Experience shows us it is wrong, but as Descartes knew, we have to presuppose it anyway. For if we do not, we weaken our own power of thought, and give it to others.
Good sense isn't the best shared thing in the world, but who cares. As Kant said, there is nothing more unworthy of a philosopher than discarding ideas by pointing out how experience contradicts them. Reflecting on this, another well-known phrase linked to Descartes, "mundus est fabula," takes on new meaning. What is the case, what is factual, is of course important. But in thought, what is factual is of less importance than the manner in which we think. Do we think from ourselves, or do we think by way of other people's thoughts? The way we choose to think determines the facts that we'll encounter. As any scientist knows, method determines content. Do we choose to believe in the power of our own thought, or do we choose to grant power to others. This is the stake of the Cartesian argument. And perhaps it is the case, that we have to believe that good sense is the best shared thing in the world.