In normal times the rebels which every society contains are restrained by the laws, by their environment--in short, by all the usual social constraints, and therefore remain undetected. But as soon as a time of disturbance begins these constraints grow weaker, and the rebel can give a free reign to his instincts. He then becomes the accredited leader of a movement. The motive of the revolution matters little to him; he will give his life indifferently for the red flag or the white, or for the liberation of a country which he has heard vaguely mentioned. The revolutionary spirit is not always pushed to the extremes which render it dangerous. When, instead of deriving from affective or mystic impulses, it has an intellectual origin, it may become a source of progress. It is thanks to those spirits who are sufficiently independent to be intellectually revolutionary that a civilisation is able to escape from the yoke of tradition and habit when this becomes too heavy. The sciences, arts, and industries especially have progressed by the aid of such men. Galileo, Lavoisier, Darwin, and Pasteur were such revolutionaries. Although it is not necessary that a nation should possess any large number of such spirits, it is very necessary that it should possess some. Without them men would still be living in caves. The revolutionary audacity which results in discoveries implies very rare faculties. It necessitates notably an independence of mind sufficient to escape from the influence of current opinions, and a judgement that can grasp, under superficial analogies, the hidden realities. This form of revolutionary spirit is creative, while that examined above is destructive. The revolutionary mentality may, therefore, be compared to certain physiological states in the life of the individual which are normally useful, but which, when exaggerated, take a pathological form which is always hurtful. 2. The Criminal Mentality. All the civilised societies inevitably drag behind them a residue of degenerates, of the unadapted, of persons affected by various taints. Vagabonds, beggars, fugitives from justice, thieves, assassins, and starving creatures that live from day to day, may constitute the criminal population of the great cities. In ordinary times these waste products of civilisation are more or less restrained by the police. During revolution nothing restrains them, and they can easily gratify their instincts to murder and plunder. In the dregs of society the revolutionaries of all times are sure of finding recruits. Eager only to kill and to plunder, little matters to them the cause they are sworn to defend. If the chances of murder and pillage are better in the party attacked, they will promptly change their colours.