Aristotle in his book of philosophy, Nicomachean Ethics reminds us that virtues are dispositions of the soul to achieve happiness. In his book II, art 6, he teaches: "If all science fulfills its function well, looking at the middle term and directing its works to it (thence, what is usually said about of the excellent works, which can not be removed or added nothing), and if on the other hand, virtue, as nature is more accurate and better than all art, will have to tend to the middle term".
Aristotle give to us a philosophycal definition of virtue: "a middle between two vices, one by excess and one by default."
Aristotle explains how to find the middle term, book II, art 7: "in relation to fear and audacity, courage is the middle term ... In the domain of pleasures and pains the middle term is moderation and the excess the intemperance In relation to giving and receiving money, the middle term is liberality, the excess and the defect are, respectively, prodigality and stinginess". Prodigal is the one that exceeds the spending and stingy who does not spend.
There are another middle terms, for example: modesty is the value between shyness and shamelessness. Meekness is the virtue between the indolence and irritability.
We need to understand the extreme, to understand the middle, to find a virtue is necessary first to understand its excesses or vices. That is why Aristotle affirms an inspiring sentence: Sometimes to know a virtue is necessary first to know its opposite.
Virtue, live moderately, as Aristotle said dispose us not only for happiness but all the good things in life like friends, peace, well being. Virtue is necessary for the formation of the character and avoid the suffering that vices and excesses generates. We know virtues throught vices and viceversa.