in short

This literary Folly is an attraction, but hardly a fascination. It governs all that is facile, joyous or light-hearted in the world. It is a madness that causes men to make merry and rejoice, just as it gave the classical gods 'Spirit and Youthfulness, Bacchus, Silenus and that quiet guardian of gardens.' Madness was a shiny, reflective surface, with no dark secrets lurking below.
  Undoubtedly, it did have links with some of the darker byways of knowledge. The first canto of Brant's Narrenschiff tells of books and bookmen, and in the engraving that illustrates the passage in the Latin edition of 1497, perched on his throne and surrounded by books, a Master with his doctoral bonnet can be seen, and behind his hat is a fool's cap sown with bells. In his great dance of the mad, Erasmus reserved pride of place for the learned. After the grammarians come the poets, the rhetoricians and the writers; then the lawyers and the philosophers 'venerable with their beards and robes', followed at last by a hurried multitude of theologians. But if knowledge is important for madness, it is not because madness might hold some vital secrets: on the contrary, it is the punishment for useless, unregulated knowledge. If it is the truth about knowledge, then all it reveals is that knowledge is derisory, and that rather than addressing the great book of experience, learning has become lost in the dust of books and in sterile discussions, knowledge made mad by an excess of false science.

Oh ye men of science, who bear great names,
Look back at the ancient fathers, learned in law.
They did not weigh dogmas in shining white books,
But fed their thirsty hearts with natural skill.

As in the theme so long familiar in popular satire, madness appears here as the comic punishment of knowledge and its ignorant presumption.
  For in a general manner madness here is not linked to the world and its subterranean forms, but rather to man and his frailties, his dreams and illusions. The dark cosmic forces at work in madness that are so apparent in the work of Bosch are absent in Erasmus. Madness no longer lies in wait for man at every crossroads; rather, it slips into him, or is in fact a subtle relationship that man has with himself. In Erasmus, the mythological personification of Madness is no more than literary artifice. Here, rather than madness, there are only follies, human forms of madness: 'I considered that as many statues have been set up for me as there men who display, sometimes unwillingly, a living image of me.' When he turns his attention to even the wisest and best-governed cities, 'wherever you look they abound in so many forms of folly, and they think up so many new ones from day to day, a thousand Democritus's would not be enough to laugh at them.' Madness is only in each man, as it lies in the attachments that men have to themselves, and the illusions that they entertain about themselves. Philautia or self-love is the first among the figures that Folly leads on its dance, but that is because these two forms are linked to each other above all others; an attachment to oneself is the first sign of madness, and it is through that attachment to oneself that man takes error for truth, lies for reality, violence and ugliness for beauty and justice. ... In this imaginary adhesion to the self, madness is born like a mirage. From now on, the symbol of madness was to be a mirror, which reflected nothing real, but secretly showed the presumptuous dreams of all who gazed into it to contemplate themselves. Madness here was not about truth or the world, but rather about man and the truth about himself that he can perceive. ...
  In the domain of literature and philosophy, the experience of madness in the fifteenth century takes on above all the appearance of a moral satire. There is little there to recall the overwhelming threat of invasion that haunted the imagination of the painters. On the contrary, care was taken to neutralise that threat: literature and philosophy are quite simply talking of a different experience. Erasmus turns attention away from that madness 'sent up from the underworld by the avenging Furies whenever they dart forth their serpents', for the Folly that he set out to praise was of a different order. ... A world of calm, without secret, that is easily mastered and fully displays its naive reductions to the eyes of the wise, who keep their distance easily through laughter. Whereas Bosch, Brueghel and Durer were earthly spectators pulled into the madness that they saw seething around them, Erasmus observes it from a distance that ensures that he is never drawn in. Like an Olympian God he observes it from on high, and if he sings its praises, it is because his laughter is the inexhaustible good humour of the gods themselves. For the madness of man is a sight for divine eyes:

In brief, if you could look down from the moon, as Menippus once did, and see the innumerable broils of mortals, you would think you were looking at a great cloud of flies or gnats quarrelling among themselves, warring, plotting, plundering, playing, frisking, being born, declining, dying. It is downright incredible what tumults, what tragedies can be stirred up by such a tiny creature, so frail and short-lived.

Madness is no longer the familiar strangeness of the world, but a spectacle well known to the observer from outside; not a figure of the cosmos, but merely of the order of the aevum.

... Conversely, many figures of moral rhetoric are illustrated in a direct manner in the cosmic images of madness: Bosch's famous doctor is far more insane than the patient he is attempting to cure, and his false knowledge does nothing more than reveal the worst excesses of a madness immediately apparent to all but himself. For his contemporaries and for the generations that followed, Bosch was above all a moralist, and his work was a series of moral lessons. His figures were born of this world, but they demonstrated the monstrous contents of the human heart. 'The difference between the paintings of this man and those of others is that others usually portray man as he appears from the outside: Bosch alone dares paint them as they are within.'

... By contrast, in Brant, Erasmus and the whole humanist tradition, madness is confined to the universe of discourse. There it becomes ever more refined, more subtle, and is slowly disarmed. It changes scale: born in the hearts of men, it rules and disrupts their conduct; when it rules cities, the calm truth of things and nature herself are unaware of its existence. It disappears fast when essential issues like life and death or justice and truth appear. It may hold every man in its control, but its reign is narrow and relative as its mediocre truth is constantly unmasked by the penetrating gaze of the savant. For such men of science, it becomes a mere object, and in the worst possible manner, as it often winds up an object of ridicule: they tamed it by the act of praising it. Even if madness was wiser than science, it would still find itself obliged to bow down before wisdom itself, the condition of its being. Now and then it might have the last word, but it never was the last word about the truth of the world, for its self-justificatory discourse is bound up with a critical consciousness of man.
  This conflict between critical consciousness and tragic experience underlies all that was felt and formulated on the theme of madness at the beginning of the Renaissance. But it was short-lived ... it is a question of the ever-increasing importance that the Renaissance accorded to one of the elements in the system—the vision of madness as an experience within the domain of language, where man was confronted with his moral truth, the laws of human nature and human truth. In short, the critical consciousness of madness was increasingly brought out into the light, while its more tragic components retreated ever further into the shadows, soon to almost vanish entirely.

Michel Foucault, History of Madness (2006:22-27)

a basic pessimism about the majority of human beings

Christianity took from its great competitor a doctrine that may bring you to despair when you study the history of trinitarian and christological thought. The dogmatic development of Christianity cannot be understood without it.
  Logos means "word". But it also refers to the meaning of a word, the reasonable structure which is indicated by a word. Therefore, Logos can also mean the universal law of reality. This is what Heraclitus meant by it ... The Logos for him was the law which determines the movements of all reality.
  For the Stoics the Logos was the divine power which is present in everything that is. There are three aspects to it, all of which become extremely important in the later development. The first is the law of nature. The Logos is the principle according to which all natural things move. It is the divine seed, the creative power, which makes anything what it is. And it is the creative power of movement of all things. Secondly, Logos means the moral law. With Immanuel Kant we could call this the "practical reason", the law which is innate in every human being when he accepts himself as a personality, with the dignity and greatness of a person. When we see the term "natural law" in classical books, we should not think of physical laws, but of moral laws. For example, when we speak of the "rights of man" as embodied in the American Constitution, we are speaking of natural law.
  Thirdly, Logos also means man's ability to recognize reality; we could call it "theoretical reason". It is man's ability to reason. Because man has the Logos in himself, he can discover it in nature and history. From this it follows for Stoicism that the man who is determined by the natural law, the Logos, is the logikos, the wise man. But the Stoics were not optimists. They did not believe that everybody was a wise man. Perhaps there were only a few who ever reached this ideal. All the others were either fools or stood somewhere between the wise and the foolish. So Stoicism held a basic pessimism about the majority of human beings.
  Originally the Stoics were Greeks; later they were Romans. Some of the most famous Stoics were Roman emperors ... They applied the concept of the Logos to the political situation for which they were responsible. The meaning of the natural law was that every man participates in reason by virtue of the fact that he is a human being. From this basis they derived laws far superior to many that we find in the Christian Middle Ages. They gave universal citizenship to every human being because everyone potentially participates in reason. Of course, they did not believe that people were actually reasonable, but they presupposed that through education they could become so. Granting Roman citizenship to all citizens of the conquered nations was a tremendous equalizing step. Women, slaves, and children, who were regarded as inferior beings under the old Roman law, became equalized by the laws of the Roman emperors. This was not done by Christianity but by the Stoics, who derived this idea from their belief in the universal Logos in which everyone participates. (Of course, Christianity holds the same idea on a different basis: all human beings are the children of God the Father.) Thus the Stoics conceived of the idea of a state embracing the whole world, based on the common rationality of everybody. This was something which Christianity could take up and develop. The difference was that the Stoics did not have the concept of sin. They had the concept of foolishness, but not sin. Therefore, salvation in Stoicism is a salvation through reaching wisdom. In Christianity salvation is brought about by divine grace. These two approaches are in conflict with each other to the present day.

Paul Tillich, A History of Christian Thought (1968: 7-8)