Along with the phrase "Know thyself," these words were carved into the columns of the Temple of Apollo at Delphi, where the famous Oracle presided as a symbol of wisdom in ancient Greece. They represent a fitting description of the virtue of temperance (Greek = sophrosyne), which Aristotle defines in the Nicomachean Ethics as having appetites "for the right things, in the right ways, at the right times." The temperate person's appetites are under the control of his reason.
Temperance is one of the "four cardinal virtues" along with prudence, justice, and fortitude. Similar to fortitude, temperance helps one conquer obstacles to doing what is prudent and just, i.e., what is good. Whereas fortitude helps one conquer fear of bodily harm, the virtue of temperance helps one conquer attachments to bodily pleasure. In particular, Aristotle held that temperance deals with those pleasures that result from the senses of touch and taste.
Of the four virtues, temperance is the one most focused on self, though it is a focus on self for the sake of being more just toward others. When we're consumed with our bodily needs, we're less able to "give each his due," which is the definition of justice.
The temperate person is one who consistently exercises the "mean"—the right path between opposite extremes.
On one side of temperance are the extremes of gluttony and lust. Aristotle definesgluttony as "eating indiscriminately or drinking until we are too full." In all animals, appetite for food and drink exists so that humans will fulfill a lack they need to survive. The enjoyment that comes from doing this—except, of course, when it's really bad food—is a nice encouragement to get us to eat and drink.
Going beyond satisfying that need by overeating or overindulging in drink gets into the territory of gluttony. When we overeat, we're not using food in a way that is in harmony with reason, i.e., eating only enough to nourish ourselves and satisfy our hunger. And, since reason is the main characteristic that separates man from the beasts, our failure to use reason in regard to food makes us more like the beasts (except, even they know when to stop eating).
Interestingly, some have also seen "picky eating" as a form of gluttony. After all, like someone who overeats, a picky eater does not use food and drink in the way he is supposed to. A temperate person is able to eat the food that is put in front of him (allowing for exceptions), and be thankful for it, because it satisfies his hunger and enables him to pursue other activities. An intemperate person, however, has such a particular palate that he will only eat a limited number of foods of a certain type or quality.
Alcohol is most famously associated with overindulgence in drink, so much so that the 18th and 19th century movements to discourage excessive alcohol consumption were called "temperance movements." A number of these movements turned into calls for complete abstention from alcohol and forbiddance of it by law. But, while some traditions may consider it virtuous to avoid alcohol altogether, such avoidance is not really temperance. Temperance is about moderation of certain desires at certain times; not necessarily about fleeing certain desires at all times. Also, because a virtuous character is developed through free choices, making a substance illegal to some extent takes away the ability to develop the virtue of temperance—something Mayor Bloomberg in NYC may want to consider.
The other extreme of lust consists of engaging in sexual behavior (in thought or action) outside of the proper context of such behavior. Sexual behavior and the pleasure that results from it is a good, but only when engaged in "in the right ways, at the right times," which for much of Western tradition, has been within the context of marriage.
In Western society today, we're much more comfortable with defining the boundaries of proper eating and drinking than we are with defining the boundaries of proper sexual behavior. The "Sexual Revolution" of the 1960s was proclaimed as a triumph of human freedom that unshackled men and women from the chains of past sexual mores, and widened the contexts in which sexual behavior was considered acceptable. If one subscribes to Aristotle's understanding of temperance, however, the Sexual Revolution might have in fact caused people to become more enslaved to their sexual appetites. Many today suffer from addictions to sex or pornography, or suffer from the consequences of engaging in sex outside of marriage. Consider, for instance, the number of children who are brought up in single-parent families, or the number of relationships that have been broken up by adultery.
Obviously, food, drink, and sex are not the only things one can be intemperate with. Some may have addictions to TV or video games, engaging in these activities for hours on end while they should be doing other things. A lot of us are also guilty of being intemperate with our smart phones. How many of us repeatedly check our email, Facebook, or Twitter accounts when we should be focusing our attention on work or conversation? While reading is usually considered more commendable than watching TV, one can also be intemperate with books. Perhaps you are someone who frequently gets so engrossed in a book that you can't put it down and irresponsibly ignore other duties.
While those who overindulge in bodily pleasures represent one pole of the violation of temperance, those who do not care for bodily pleasures represent the other pole. Aristotle, however, doesn't spend much time talking about these people, simply because "People who are deficient in pleasures and enjoy them less than is right are not found very much." They also wouldn't be very much fun to hang out with.
It is important to remember that virtues and vices are behaviors that have become more set in stone. So, one who is temperate consistently and happily exercises control over his bodily desires, and one who is intemperate consistently and stubbornly fails to exercise such control.
Aristotle had different words to describe those who exercise self-control, but only with much struggle, and those who occasionally slip up: "continent" and "incontinent."
For instance, in George Orwell's novel 1984, young Winston swipes a bar of chocolate from the clutches of his starving mother and sister. One wouldn't label him "intemperate"; rather, he simply had a moment of incontinence. Or, perhaps you binged on watching season 4 of Arrested Development when it came out, or maybe you picked up The Hunger Games and let the housecleaning slide for a couple of days while you read all 3 books. Such actions don't necessarily mean that you're intemperate; they may just mean that you're incontinent.