Based on Mishnah Torah I: Book of Knowledge, the Laws of Character, Ch. 1
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The Rambam's Own Words:
Each and every man possesses many character traits. Each trait is very different and distant from the others.
One type of man is wrathful; he is constantly angry. [In contrast,] there is the calm individual who is never moved to anger, or, if at all, he will be slightly angry, [perhaps once] during a period of several years.
There is the prideful man and the one who is exceptionally humble. There is the man ruled by his appetites - he will never be satisfied from pursuing his desires, and [conversely,] the very pure of heart, who does not desire even the little that the body needs.
There is the greedy man, who cannot be satisfied with all the money in the world, as [Ecclesiastes 5:9] states: "A lover of money never has his fill of money." [In contrast,] there is the man who puts a check on himself; he is satisfied with even a little, which is not enough for his needs, and he does not bother to pursue and attain what he lacks.
There is [the miser,] who torments himself with hunger, gathering [his possessions] close to himself. Whenever he spends a penny of his own, he does so with great pain. [Conversely,] there is [the spendthrift,] who consciously wastes his entire fortune.
All other traits follow the same pattern [of contrast]. For example: the overly elated and the depressed; the stingy and the freehanded; the cruel and the softhearted; the coward and the rash. and the like.
Between each trait and the [contrasting] trait at the other extreme, there are intermediate points, each distant from the other.
With regard to all the traits: a man has some from the beginning of his conception, in accordance with his bodily nature. Some are appropriate to a person's nature and will [therefore] be acquired more easily than other traits. Some traits he does not have from birth. He may have learned them from others, or turned to them on his own. This may have come as a result of his own thoughts, or because he heard that this was a proper trait for him, which he ought to attain. [Therefore,] he accustomed himself to it until it became a part of himself.
The two extremes of each trait, which are at a distance from one another, do not reflect a proper path. It is not fitting that a man should behave in accordance with these extremes or teach them to himself.
If he finds that his nature leans towards one of the extremes or adapts itself easily to it, or, if he has learned one of the extremes and acts accordingly, he should bring himself back to what is proper and walk in the path of the good [men]. This is the straight path.