Lewis talks about the four cardinal virtues: prudence, temperance, justice and fortitude. Cardinal means “hinge”, so these virtues hold the other virtues together (except the theological virtues). Christians are called to practice them. They are necessary for the formation of a good character.
Christians should “clean up” their inner selves. They should “harmonize” their individual persons. They should set themselves aright. The cardinal virtues help us do that.