According to Aristotle, virtue primarily involves the use of a person's rationality. Rationality and happiness are activities of the soul, and virtue is the excellence of these activities. Humans are the only life forms that have souls, the source of rationality. Thus, humans have a duty to always use their intellect. Three things are found in the soul: emotions, capacities, and characteristics. Emotions are things humans feel, like anguish or happiness, that are followed by pain or pleasure. Capacities are a person's ability or capacity to experience or express something, for example, being able to feel the presence of God is a capacity. Since people are not considered good or bad based on their emotions, virtue cannot be an emotion, because virtue is a complete good. Virtue is not a capacity either, because virtue involves choice, not abilities. "Virtues are neither feelings nor capacities" (Nicomachean Ethics 1106a 12). Therefore, virtue is a characteristic of a person that renders good, of which it is the excellence and causes it to perform its function well. In other words, a person with a good character has virtue. The aim of all human action is for the good, and any virtuous act is a good act. A virtuous act must be based on rationality and acted on after deliberation by the individual. Therefore, a virtuous individual must be knowledgeable about what is good, must only make choices after careful deliberation, and must be a good judge of proper action. These virtuous characteristics come from experience, training, an environment conducive to learning, a love of rationality, and good habits developed from constant practice. Pleasure causes humans to do base actions. Pain keeps us from doing noble actions. Virtue involves maintaining a balance between and pleasure: "For virtue of character is about pleasures and pains" (Nicomachean Ethics 1104b3-10), Aristotle stressed that this moderation of pain and pleasure is an extremely important aspect of virtue.