The cardinal virtues are the four principal moral virtues. The English word cardinal comes from the Latin word cardo, which means "hinge." All other virtues hinge on these four: prudence, justice, fortitude, and temperance.​ Plato first discussed the cardinal virtues in the Republic, and they entered into Christian teaching by way of Plato's disciple Aristotle. Unlike the theological virtues, which are the gifts of God through grace, the four cardinal virtues can be practiced by anyone; thus, they represent the foundation of natural morality.

Prudence: The First Cardinal Virtue Personification of Prudence - Gaetano Fusali. Wikimedia Commons St. Thomas Aquinas ranked prudence as the first cardinal virtue because it is concerned with the intellect. Aristotle defined prudence as recta ratio agibilium, "right reason applied to practice." It is the virtue that allows us to judge correctly what is right and what is wrong in any given situation. When we mistake the evil for the good, we are not exercising prudence—in fact, we are showing our lack of it. Because it is so easy to fall into error, prudence requires us to seek the counsel of others, particularly those we know to be sound judges of morality. Disregarding the advice or warnings of others whose judgment does not coincide with ours is a sign of imprudence.

Justice: The Second Cardinal Virtue Allegory of Justice detail of the mosaic floor at the Basilica of San Savino, Piacenza, Emilia-Romagna, Italy, 12th century. DEA Picture Library / Getty Images Justice, according to Saint Thomas, is the second cardinal virtue, because it is concerned with the will. As Fr. John A. Hardon notes in his Modern Catholic Dictionary, it is "the constant and permanent determination to give everyone his or her rightful due." We say that "justice is blind," because it should not matter what we think of a particular person. If we owe him a debt, we must repay exactly what we owe. Justice is connected to the idea of rights. While we often use justice in a negative sense ("He got what he deserved"), justice in its proper sense is positive. Injustice occurs when we as individuals or by law deprive someone of that which he is owed. Legal rights can never outweigh natural ones.

Fortitude: The Third Cardinal Virtue Allegory of the Fortress; detail of the mosaic floor at the Basilica of San Savino, Piacenza, Emilia-Romagna, Italy, 12th century. DEA / A. DE GREGORIO / Getty Images The third cardinal virtue, according to St. Thomas Aquinas, is fortitude. While this virtue is commonly called courage, it is different from what much of what we think of as courage today. Fortitude allows us to overcome fear and to remain steady in our will in the face of obstacles, but it is always reasoned and reasonable; the person exercising fortitude does not seek danger for danger's sake. Prudence and justice are the virtues through which we decide what needs to be done; fortitude gives us the strength to do it. Fortitude is the only one of the cardinal virtues that is also a gift of the Holy Spirit, allowing us to rise above our natural fears in defense of the Christian faith.