The Instructor 2.5.46

Laughter itself must be bridled too. For when it is let out in the right way it shows decorum, but when it goes otherwise it shows licentiousness. In a word, whatever is natural to human beings must not be taken away from them, but rather given a fitting measure and time. For because the human being is an animal that laughs, it does not follow that he should laugh at everything; the horse, too, being an animal that neighs, does not neigh at everything. As rational animals we must tune ourselves in good proportion, relaxing melodiously the severity and overstrain of our seriousness, not loosening it unmelodiously. The decorous relaxing of the face in harmony, as of an instrument, is called a smile (this is how cheerfulness is reflected on the face), the laughter of the self-controlled. But an unmelodious loosening of the face, if it happens in women, is called giggling, and is the laughter of a harlot; if in men, guffawing, and this is the laughter of the suitors, and insolent. “A fool raises his voice in laughter,” says Scripture, “but a shrewd man will scarcely smile quietly.”