320. Sensations considered as purely subjective do not meet the wants of sensitive life. We must be sure that our sensations correspond to an external world, real and true, not phenomenal. Men do not ordinarily possess either the capacity or the time requisite to investigate the philosophical questions of the existence of bodies, and to decide for or against Berkeley and his followers. What is necessary is perfect certainty that bodies do exist, that sensations have an external object in reality. All men have this certainty when they assent with an irresistible force to the objectiveness of ideas, that is, to the existence of bodies. 321. Faith in human authority furnishes us with another case of this wonderful instinct. Both the individual and society require faith: without it society and family would be impossible; the individual would be condemned to isolation, and, therefore, to death. The speech of man, the human race itself, would disappear were it not for faith. This belief has distinct grades according to different circumstances, but it always exists; man is inclined, by a natural instinct, to believe his fellow man. When many men speak, and none raise their voices against them, the force of this inclination increases in the same proportion as the number of witness, until it becomes irresistible. Who doubts the existence of Rome? And yet, the greater part of us only know it upon the authority of other men. What foundation has faith in human authority? Most men are ignorant of the philosophical reasons which may be assigned; but their faith is not therefore less lively than that of philosophers. But why is this so? Because there is a necessity, and at its side an instinct to be satisfied. Man must believe in man, and he believes. And here note well, that the greater the necessity, the greater the faith. The very ignorant and imbecile believe all that is told them; they make other men their guide, and blindly follow them. The tender child, knowing nothing, abandons itself to the absolute belief of the greatest marvels: the word of those around is to it an invaluable criterion of truth. 322. Besides the cases of first intellectual and moral principles, the objectiveness of ideas and sensations, the weight of human authority, man must give his instantaneous assent to certain truths, which, although he might, had he the time, demonstrate, he cannot now, because so suddenly are they presented to him, that they exact an immediate formation of judgment, and, sometimes, also action. In all these cases, there is a natural inclination impelling us to assent.