“The questions raised by the desire to know are in principle all answerable by common-sense experience and common-sense reasoning,” Hannah Arendt wrote in her brilliant treatise on thinking vs. knowing and the crucial difference between truth and meaning. “But the questions raised by thinking and which it is in reason’s very nature to raise — questions of meaning — are all unanswerable by common sense and the refinement of it we call science.”

What kind of reasoning, then, can we develop in order not only to inoculate ourselves against unreason, not only to arrive at truth, but to access meaning? More than that, in an age of instant opinions blind to the fact that the hunger for certainty often starves us of truth, how are we to train ourselves to shed the blinders of opinion and attune our organs of insight to unclouded truth?

Four centuries ago, the great French philosopher, scientist, and mathematician René Descartes (March 31, 1596–February 11, 1650) set out to answer these questions in a 1637 treatise on “the method for conducting one’s reason well,” included in his Philosophical Essays and Correspondence (public library). Building on the twelve tenets of critical thinking he had penned a decade earlier, in this part-autobiographical, part-philosophical work Descartes delineates how he arrived at his famous I think therefore I am dictum and illustrates the discipline of cultivating a wakeful, considerate mind honed for discerning truth from unreason, a mind sensitive to its own propensity for self-deception and therefore compassionate to even the most deluded others.

Descartes begins by considering the nature of common sense and the cause of our divergent opinions:

Good sense is the best distributed thing in the world, for everyone thinks himself to be so well endowed with it that even those who are the most difficult to please in everything else are not at all wont to desire more of it than they have. It is not likely that everyone is mistaken in this. Rather, it provides evidence that the power of judging well and of distinguishing the true from the false (which is, properly speaking, what people call “good sense” or “reason”) is naturally equal in all men, and that the diversity of our opinions does not arise from the fact that some people are more reasonable than others, but solely from the fact that we lead our thoughts along different paths and do not take the same things into consideration. For it is not enough to have a good mind; the main thing is to apply it well. The greatest souls are capable of the greatest vices as well as of the greatest virtues. And those who proceed only very slowly can make much greater progress, provided they always follow the right path, than do those who hurry and stray from it.

He reflects on the path of his own steadfast progress toward reason:

For myself, I have never presumed that my mind was in any respect more perfect than that of ordinary men.

[…]

But I shall have no fear of saying that I think I have been rather fortunate to have, since my youth, found myself on certain paths that have led me to considerations and maxims from which I have formed a method by which, it seems to me, I have the means to increase my knowledge by degrees and to raise it little by little to the highest point which the mediocrity of my mind and the short duration of my life will be able to allow it to attain. For I have already reaped from it such a harvest that, although I try, in judgments I make of myself, always to lean more on the side of diffidence than of presumption, and although, looking with a philosopher’s eye at the various actions and enterprises of all men, there is hardly one of them that does not seem to me vain and useless, I cannot but take immense satisfaction in the progress that I think I have already made in the search for truth, and I cannot but envisage such hopes for the future that if, among the occupations of men purely as men, there is one that is solidly good and important, I dare to believe that it is the one I have chosen.

Although he was “nourished on letters” since childhood but “delighted most of all in mathematics because of the certainty and the evidence of its reasonings,” Descartes awakened early to the lacuna between learning and reasoning, becoming aware that if so many expert opinions could exist about the selfsame subject, it was quite probable that many of them were incorrect. Thus he realized that the most reliable tool of evaluating truth is a well-trained critical mind of one’s own. But contrary to hasty misconception, Descartes was no reclusive scholar who simply theorized these precepts in the isolated comfort of an ivory tower. Rather, he wrested them from what he calls “the book of the world,” in consonance with Werner Herzog’s insistence that all work aimed at creative breakthrough “must have experience of life at its foundation,” and then continually tested them against reality as he interacted with that world. He recounts:

As soon as age permitted me to emerge from the supervision of my teachers, I completely abandoned the study of letters. And resolving to search for no knowledge other than what could be found within myself, or else in the great book of the world, I spent the rest of my youth traveling, seeing courts and armies, mingling with people of diverse temperaments and circumstances, gathering various experiences, testing myself in the encounters that fortune offered me, and everywhere engaging in such reflection upon the things that presented themselves that I was able to derive some profit from them. For it seemed to me that I could find much more truth in the reasonings that each person makes concerning matters that are important to him, and whose outcome ought to cost him dearly later on if he has judged badly, than in those reasonings engaged in by a man of letters in his study, which touch on speculations that produce no effect and are of no other consequence to him except perhaps that, the more they are removed from common sense, the more pride he will take in them, for he will have to employ that much more wit and ingenuity in attempting to render them plausible. And I have always had an especially great desire to learn to distinguish the true from the false, in order to see my way clearly in my actions, and to go forward with confidence in this life.

It is true that, so long as I merely considered the customs of other men, I found hardly anything there about which to be confident, and that I noticed there was about as much diversity as I had previously found among the opinions of philosophers. Thus the greatest profit I derived from this was that, on seeing many things that, although they seem to us very extravagant and ridiculous, do not cease to be commonly accepted and approved among other great peoples, I learned not to believe anything too firmly of which I had been persuaded only by example and custom; and thus I little by little freed myself from many errors that can darken our natural light and render us less able to listen to reason. But after I had spent some years thus studying in the book of the world and in trying to gain some experience, I resolved one day to study within myself too and to spend all the powers of my mind in choosing the paths that I should follow.

In a sentiment which John Cowper Powys would echo centuries later in his meditation on the crucial difference between being educated and being cultured, Descartes contrasts the value of such introspective critical thinking with that of rote learning:

Book learning, at least the kind whose reasonings are merely probable and that do not have any demonstrations, having been composed and enlarged little by little from the opinions of many different persons, does not draw nearly so close to the truth as the simple reason-ings that a man of good sense can naturally make about the things he encounters… Because we were all children before being men and because for a long time it was necessary for us to be governed by our appetites and our teachers (which were frequently in conflict with one another, and of which perhaps neither always gave us the best advice), it is nearly impossible for our judgments to be as pure or as solid as they would have been if we had had the full use of our reason from the moment of our birth and if we had always been guided by it alone.

But he offers one crucial disclaimer, inviting a sensitivity to nuance in distinguishing between expertise and authority, one being the product of objectively superior knowledge and the other of accepted order or unquestioned custom. Expertise, he cautions, is not to be dismissed by those who lack it in the very subject they are trying to evaluate:

The single resolution to rid oneself of all the opinions to which one has heretofore given credence is not an example that everyone ought to follow; and the world consists almost exclusively of two kinds of minds for whom it is not at all suitable. First, there are those who, believing themselves more capable than they are, are unable to avoid being hasty in their judgments or have enough patience to conduct all their thoughts in an orderly manner; as a result, if they have once taken the liberty of doubting the principles they had accepted and of straying from the common path, they could never keep to the path one must take in order to go in a more straightforward direction, and they would remain lost all their lives. Second, there are those who have enough reason or modesty to judge that they are less capable of distinguishing the true from the false than certain others by whom they can be instructed; such people should content themselves more with following the opinions of these others than with looking for better ones themselves.

He returns to the question of what gives rise to divergent opinions and how irrelevant opinions are, at bottom, in the conquest of truth:

I had recognized in my travels that all those who have sentiments quite contrary to our own are not for that reason barbarians or savages, but that many of them use their reason as much as or more than we do… One and the same man with the very same mind, were he brought up from infancy among the French or the Germans, would become different from what he would be had he always lived among the Chinese or the cannibals.

Predating Kierkegaard’s assertion that “truth always rests with the minority” by two centuries, Descartes adds:

Thus it is more custom and example that persuades us than any certain knowledge; and yet the majority opinion is worthless as a proof of truths that are at all difficult to discover, since it is much more likely that one man would have found them than a whole multitude of people.

In the remainder of the treatise, Descartes goes on to outline the four rules he devised for himself to help him efficiently and lucidly evaluate what he encounters, and emerge with truth. Complement his Philosophical Essays and Correspondence with his reflections on the relationship between fear and hope, the cure for indecision, and how to acquire nobility of spirit, then revisit Hegel on the peril of fixed opinions and Karl Popper on the vital difference between truth and certainty.