The theory of the mean is one of Aristotle's best-known pieces of ethical thinking. It can be found in his book The Nicomachean Ethics. It is at once strikingly simple and fiendishly difficult. In a nutshell, Aristotle said that virtues are a point of moderation between two opposite vices. For instance, the virtue courage lies between the two vices of cowardice and recklessness. Recklessness is too much confidence and not enough fear, cowardice is too much fear and not enough confidence, courage is just the right amount of both.
This can be expanded to most virtues and vices. Some other means that Aristotle laid out were temperance (or self-control), which lies between self-indulgence and a lack of sensitivity to your own needs, and modesty which is between bashfulness and vanity.
Some of the “virtues” may seem a bit odd to those brought up with a post-Christian worldview. Aristotle thinks you should have the “right amount” of pride, depending on your worth, and that humility is a deficiency of proper pride, and therefore a vice. He also thinks you should feel anger to the right extent – too little anger when someone dents your car and you don't have a proper respect for either your possessions or your own dignity.
As you may have noticed, the mean is about the proper emotional response to situations, rather than the proper actions. For Aristotle, virtue came from character. It is the character that makes you do the good deeds that is virtuous, not the deeds themselves. This school of moral thought is still around today and is called Virtue Ethics.
There are some problems to get over if you're thinking of living your life according to the Golden Mean, however. Firstly, how do you find the mean? Aristotle makes it clear that he is not talking about a mathematical middle, but the perfect intermediate point with regard to ourselves. To adapt an example Aristotle gives, if a baby only needs a teacupful of food each day to thrive, and a fully grown adult needs three square meals, that doesn't mean the ideal amount is halfway in between. If you're old enough to be reading this, you probably need an amount a lot closer to the three square meals than the teacup.
The same thing goes with the mean. The perfect point might be closer to excess, or closer to deficiency. Courage is closer to recklessness than cowardice, and self-control is closer to lack of sensitivity to one's needs than to self indulgence. This makes it hard to decide where the mean actually is, especially as everyone has a different idea of what is too modest, too indulgent, etc.
Even if you do manage to find the mean, it's not enough just to follow it. You can do good deeds but unless you have a good character yourself, Aristotle says the actions won't be truly virtuous. You have to keep doing the right thing so that your character can become good, through training, and only then will your good deeds actually be good!
Following the mean would take a lifetime to perfect, but the idea of a virtue being the perfect point on a line between two opposite vices is undeniably interesting. It can be applied to just about any aspect of life and may provide fresh insights into why bad actions are bad, and what it means to be good.