It takes Self-Control

Thus character exhibits itself in self-control of speech as much as in anything else. The wise and forbearant man will restrain his desire to say a smart or severe thing at the expense of another's feelings; while the fool blurts out what he thinks, and will sacrifice his friend rather than his joke. "The mouth of a wise man," said Solomon, "is in his heart; the heart of a fool is in his mouth."

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We have heard men of great experience say that they have often regretted having spoken, but never once regretted holding their tongue. "Be silent," says Pythagoras, "or say something better than silence." "Speak fitly," says George Herbert, "or be silent wisely." St. Francis de Sales, whom Leigh Hunt styled "the Gentleman Saint," has said: "It is better to remain silent than to speak the truth ill-humouredly, and so spoil an excellent dish by covering it with bad sauce." Another Frenchman, Lacordaire, characteristically puts speech first, and silence next. "After speech," he says, "silence is the greatest power in the world." Yet a word spoken in season, how powerful it may be! As the old Welsh proverb has it, "A golden tongue is in the mouth of the blessed."

Source: Shared from the book "Character", by Samuel Smiles