It is to the press we owe the possibility of spreading those publications which the emergency of the moment, or the transient fluctuations of opinion, may require, and of interesting thereby in any question, treated in a single point of view, whole communities of men reading and understanding the same language.

All those means which render the progress of the human mind more easy, more rapid, more certain, are also the benefits of the press. Without the instrumentality of this art, such books could not have been multiplied as are adapted to every class of readers, and every degree of instruction. To the press we owe those continued discussions which alone can enlighten doubtful questions, and six upon an immoveable basis, truths too abstract, too subtile, too remote from the prejudices of the people or the common opinion of the learned, not to be soon forgotten and lost. To the press we owe those books purely elementary, dictionaries, works in which are collected, with all their details, a multitude of facts, observations, and experiments, in which all their proofs are developed, all their difficulties investigated. To the press we owe those valuable compilations, containing sometimes all that has been discovered, written, thought, upon a particular branch of science, and sometimes the result of the annual labours of all the literati of a country. To the press we owe those tables, those catalogues, those pictures of every kind, of which some exhibit a view of inductions which the mind could only have acquired by the most tedious operations; others present at will the fact, the discovery, the number, the method, the object which we are desirous of ascertaining; while others again furnish, in a more commodious form and a more arranged order, the materials from which genius may fashion and derive new truths.

To these benefits we shall have occasion to add others, when we proceed to analyse the effects that have arisen from the substitution of the vernacular tongue of each country, in the room of the almost exclusive application, which had preceded, so far as relates to the sciences, of one language, the common medium of communication between the learned of all nations.

In short, is it not the press that has freed the instruction of the people from every political and religious chain? In vain might either despotism invade our schools; in vain might it attempt, by rigid institutions, invariably to six what truths shall be preserved in them, what errors inculcated on the mind; in vain might chairs, consecrated to the moral instruction of the people, and the tuition of youth in philosophy and the sciences, be obliged to deliver no doctrines but such as are favourable to this double tyranny: the press can diffuse at the same time a pure and independent light. That instruction which is to be acquired from books in silence and solitude, can never be universally corrupted: a single corner of the earth free to commit their leaves to the press, would be a sufficient security. How admist that variety of productions, amidst that multitude of existing copies of the same book, amidst impressions continually renewed, will it be possible to shut so closely all the doors of truth, as to leave no opening, no crack or crevice by which it may enter? If it was difficult even when the business was to destroy a few copies only of a manuscript, to prevent for ever its revival, when it was sufficient to proscribe a truth, or opinion, for a certain number of years to devote it to eternal oblivion, is not this difficulty now rendered impossible, when it would require a vigilance incessantly occupied, and an activity that should never slumber? And even should success attend the suppression of those too palpable truths, that wound directly the interests of inquisitors, how are others to be prevented from penetrating and spreading, which include those proscribed truths without suffering them to be perceived, which prepare the way, and must one day infallibly lead to them? Could it be done without obliging the personages in question to throw off that mask of hypocrisy, the fall of which would prove no less fatal than truth itself to the reign of error? We shall accordingly see reason triumphing over these vain efforts: we shall see her in this war, a war continually reviving, and frequently cruel, successful alike against violence and stratagem; braving the flames, and resisting seduction; crushing in turn, under its mighty hand, both the fanatical hypocrisy which requires for its dogmas a sincere adoration, and the political hypocrisy imploring on its knees that it may be allowed to enjoy in peace the profit of errors, in which, if you will take its word, it is no less advantageous to the people than to itself, that they should for ever be plunged.

We shall expose the origin and trace the history of general errors, which have more or less contributed to retard or suspend the advance of reason, and sometimes even, as much as political events, have been the cause of man’s taking a retrograde course towards ignorance.

Those operations of the mind that lead to or retain us in error, from the subtle paralogism, by which the most penetrating mind may be deceived, to the mad reveries of enthusiasts, belong equally, with that just mode of reasoning that conducts us to truth, to the theory of the development of our individual faculties; and for the same reason, the manner in which general errors are introduced, propagated, trasmitted, and rendered permanent among nations, forms a part of the picture of the progress of the human mind. Like truths which improve and enlighten it, they are the consequence of its activity, and of the disproportion that always exists between what it actually knows, what it has the desire to know, and what it conceives there is a necessity of acquiring.

It is even apparent, that, from the general laws of the development of our faculties, certain prejudices must necessarily spring up in each stage of our progress, and extend their seductive influence beyond that stage; because men retain the errors of their infancy, their country, and the age in which they live, long after the truths necessary to the removal of those errors are acknowledged.

In short, there exist, at all times and in all countries, different prejudices, according to the degree of illumination of the different classes of men, and according to their professions. If the prejudices of philosophers be impediments to new acquisitions of truth, those of the less enlighted classes retard the propagation of truths already known, and those of esteemed and powerful professions oppose like obstacles. These are the three kinds of enemies which reason is continually obliged to encounter, and over which she frequently does not triumph till after a long and painful struggle. The history of these contests, together with that of the rise, triumph, and fall of prejudice, will occupy a considerable place in this work, and will by no means form the least important or least useful part of it.

One property of Bayes’s theorem, in fact, is that our beliefs should converge toward one another—and toward the truth—as we are presented with more evidence over time. In figure 8-8, I’ve worked out an example wherein three investors are trying to determine whether they are in a bull market or a bear market. They start out with very different beliefs about this—one of them is optimistic, and believes there’s a 90 percent chance of a bull market from the outset, while another one is bearish and says there’s just a 10 percent chance. Every time the market goes up, the investors become a little more bullish relative to their prior, while every time it goes down the reverse occurs. However, I set the simulation up such that, although the fluctuations are random on a day-to-day basis, the market increases 60 percent of the time over the long run. Although it is a bumpy road, eventually all the investors correctly determine that they are in a bull market with almost (although not exactly, of course) 100 percent certainty.

In theory, science should work this way. The notion of scientific consensus is tricky, but the idea is that the opinion of the scientific community converges toward the truth as ideas are debated and new evidence is uncovered. Just as in the stock market, the steps are not always forward or smooth. The scientific community is often too conservative about adapting its paradigms to new evidence,64 although there have certainly also been times when it was too quick to jump on the bandwagon. Still, provided that everyone is on the Bayesian train,* even incorrect beliefs and quite wrong priors are revised toward the truth in the end.

When a scientist says something, his colleagues must ask themselves only whether it is true. When a politician says something, his colleagues must first of all ask, 'Why does he say it?'

It is rigid dogma that destroys truth; and, please notice, my emphasis is not on the dogma, but on the rigidity. When men say of any question, 'This is all there is to be known or said of the subject; investigation ends here,' that is death. It may be that the mischief comes not from the thinker but for the use made of his thinking by late-comers. Aristotle, for example, gave us out scientific technique ... yet his logical propositions, his instruction in sound reasoning which was bequeathed to Europe, are valid only within the limited framework of formal logic, and, as used in Europe, they stultified the minds of whole generations of mediaeval Schoolmen. Aristotle invented science, but destroyed philosophy.

At the outset do not be worried about this big question—Truth. It is a very simple matter if each one of you starts with the desire to get as much as possible. No human being is constituted to know the truth, the whole truth, and nothing but the truth; and even the best of men must be content with fragments, with partial glimpses, never the full fruition. In this unsatisfied quest the attitude of mind, the desire, the thirst—a thirst that from the soul must arise!—the fervent longing, are the be-all and the end-all.

The success of the paradigm... is at the start largely a promise of success ... Normal science consists in the actualization of that promise... Mopping up operations are what engage most scientists throughout their careers. They constitute what I am here calling normal science... That enterprise seems an attempt to force nature into the preformed and relatively inflexible box that the paradigm supplies. No part of the aim of normal science is to call forth new sorts of phenomena; indeed those that will not fit the box are often not seen at all. Nor do scientists normally aim to invent new theories, and they are often intolerant of those invented by others.

Physical concepts are free creations of the human mind, and are not, however it may seem, uniquely determined by the external world. In our endeavour to understand reality we are somewhat like a man trying to understand the mechanism of a closed watch. He sees the face and the moving hands, even hears its ticking, but he has no way of opening the case. If he is ingenious he may form some picture of a mechanism which could be responsible for all the things he observes, but he may never be quite sure his picture is the only one which could explain his observations. He will never be able to compare his picture with the real mechanism and he cannot even imagine the possibility or the meaning of such a comparison. But he certainly believes that, as his knowledge increases, his picture of reality will become simpler and simpler and will explain a wider and wider range of his sensuous impressions. He may also believe in the existence of the ideal limit of knowledge and that it is approached by the human mind. He may call this ideal limit the objective truth.

However, all scientific statements and laws have one characteristic in common: they are “true or false” (adequate or inadequate). Roughly speaking, our reaction to them is “yes” or “no.” The scientific way of thinking has a further characteristic. The concepts which it uses to build up its coherent systems are not expressing emotions. For the scientist, there is only “being,” but no wishing, no valuing, no good, no evil; no goal. As long as we remain within the realm of science proper, we can never meet with a sentence of the type: “Thou shalt not lie.” There is something like a Puritan's restraint in the scientist who seeks truth: he keeps away from everything voluntaristic or emotional.

Well, evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts do not go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air, pending the outcome. And humans evolved from apelike ancestors whether they did so by Darwin's proposed mechanism or by some other, yet to be discovered. .

Scientists regard debates on fundamental issues of theory as a sign of intellectual health and a source of excitement. Science is—and how else can I say it?—most fun when it plays with interesting ideas, examines their implications, and recognizes that old information might be explained in surprisingly new ways. Evolutionary theory is now enjoying this uncommon vigor. Yet amidst all this turmoil no biologist has been lead to doubt the fact that evolution occurred; we are debating how it happened. We are all trying to explain the same thing: the tree of evolutionary descent linking all organisms by ties of genealogy. Creationists pervert and caricature this debate by conveniently neglecting the common conviction that underlies it, and by falsely suggesting that evolutionists now doubt the very phenomenon we are struggling to understand.