There are sometimes in common expressions an image of what passes in the depths of all men's hearts. Among the Romans _sensus communis_ signified not only common sense, but humanity, sensibility. As we are not as good as the Romans, this word signifies among us only half of what it signified among them. It means only good sense, plain reason, reason set in operation, a first notion of ordinary things, a state midway between stupidity and intelligence. "This man has no common sense" is a great insult. "A common-sense man" is an insult likewise; it means that he is not entirely stupid, and that he lacks what is called wit and understanding. But whence comes this expression _common sense_, unless it be from the senses? Men, when they invented this word, avowed that nothing entered the soul save through the senses; otherwise, would they have used the word _sense_ to signify common reasoning? People say sometimes--"Common sense is very rare." What does this phrase signify? that in many men reason set in operation is stopped in its progress by prejudices, that such and such man who judges very sanely in one matter, will always be vastly deceived in another. This Arab, who will be a good calculator, a learned chemist, an exact astronomer, will believe nevertheless that Mohammed put half the moon in his sleeve. Why will he go beyond common sense in the three sciences of which I speak, and why will he be beneath common sense when there is question of this half moon? Because in the first cases he has seen with his eyes, he has perfected his intelligence; and in the second, he has seen with other people's eyes, he has closed his own, he has perverted the common sense which is in him. How has this strange mental alienation been able to operate? How can the ideas which move with so regular and so firm a step in the brain on a great number of subjects limp so wretchedly on another a thousand times more palpable and easy to comprehend? This man always has inside him the same principles of intelligence; he must have some organ vitiated then, just as it happens sometimes that the finest _gourmet_ may have a depraved taste as regards a particular kind of food. How is the organ of this Arab, who sees half the moon in Mohammed's sleeve, vitiated? It is through fear. He has been told that if he did not believe in this sleeve, his soul, immediately after his death, when passing over the pointed bridge, would fall for ever into the abyss. He has been told even worse things: If ever you have doubts about this sleeve, one dervish will treat you as impious; another will prove to you that you are an insensate fool who, having all possible motives for believing, have not wished to subordinate your superb reason to the evidence; a third will report you to the little divan of a little province, and you will be legally impaled.