… is from page 348 of H.L. Mencken’s, A Second Mencken Chrestomathy; specifically, it’s from Mencken’s 1926 essay “Another Long-Awaited Book”:
If the young are to be instructed at all, it seems to me that they ought to be instructed in the high human value of this toleration. They should be taught what they learn by experience in the school yard: that human beings differ enormously, one from the other, and that it is stupid and imprudent for A to try to change B. They should be taught that mutual confidence and good will are worth all the laws ever heard of, ghostly or secular, and that one man who minds his own business is more valuable to the world than 10,000 cocksure moralists.