There is sometimes in vulgar expressions an image of what passes in the heart of all men. “Sensus communis” signified among the Romans not only common sense, but also humanity and sensibility. As we are not equal to the Romans, this word with us conveys not half what it did with them. It signifies only good sense — plain, straightforward reasoning — the first notion of ordinary things — a medium between dulness and intellect. To say, “that man has not common sense,” is a gross insult; while the expression, “that man has common sense,” is an affront also; it would imply that he was not quite stupid, but that he wanted intellect. But what is the meaning of common sense, if it be not sense? Men, when they invented this term, supposed that nothing entered the mind except by the senses; otherwise would they have used the word “sense” to signify the result of the common faculty of reason?

It is said, sometimes, that common sense is very rare. What does this expression mean? That, in many men, dawning reason is arrested in its progress by some prejudices; that a man who judges reasonably on one affair will deceive himself grossly in another. The Arab, who, besides being a good calculator, was a learned chemist and an exact astronomer, nevertheless believed that Mahomet put half of the moon into his sleeve.

How is it that he was so much above common sense in the three sciences above mentioned, and beneath it when he proceeded to the subject of half the moon? It is because, in the first case, he had seen with his own eyes, and perfected his own intelligence; and, in the second, he had used the eyes of others, by shutting his own, and............