Why do conspiracy theories invade our minds ? Why does the media cover of politics tend to yield to the tabloid culture? Why do we suspect scientists ? How, in general, can imaginary, invented or even frankly false facts spread, win the support of audiences, influence political decisions, therefore shaping a part of our vision of the world ? However, was it not reasonable to expect that with the free flow of information and the increase of the level of education, democratic societies would tend to create a form of collective wisdom This invigorating essay proposes, by calling up numerous examples, to answer all these questions by showing how the conditions of our contemporary life have combined with the intimate functioning of our brain to make us fools. It is urgent to understand it.