Rambam opens by describing how human temperaments span a spectrum from extreme to extreme — e.g., rage to lethargy, elation to despair — and that health consists of the middle path (derech ha'emtza). He notes that some people are born with temperaments inclined to extremes and need special guidance.
דֵעוֹת הַרְבֵּה יֵשׁ לְכָל אֶחָד וְאֶחָד מִבְּנֵי אָדָם וְזוֹ מְשֻׁנָּה מִזּוֹ וּרְחוֹקָה מִמֶּנָּהּ בְּיוֹתֵר. יֵשׁ אָדָם שֶׁהוּא בַּעַל חֵמָה כּוֹעֵס תָּמִיד. וְיֵשׁ אָדָם שֶׁדַּעְתּוֹ מְיֻשֶּׁבֶת עָלָיו וְאֵינוֹ כּוֹעֵס כְּלָל וְאִם יִכְעַס יִכְעַס כַּעַס מְעַט בְּכַמָּה שָׁנִים. וְיֵשׁ אָדָם שֶׁהוּא גְּבַהּ לֵב בְּיוֹתֵר. וְיֵשׁ שֶׁהוּא שְׁפַל רוּחַ בְּיוֹתֵר. וְיֵשׁ שֶׁהוּא בַּעַל תַּאֲוָה לֹא תִּשְׂבַּע נַפְשׁוֹ מֵהָלֹךְ בְּתַאֲוָה. וְיֵשׁ שֶׁהוּא בַּעַל לֵב טָהוֹר מְאֹד וְלֹא יִתְאַוֶּה אֲפִלּוּ לִדְבָרִים מְעַטִּים שֶׁהַגּוּף צָרִיךְ לָהֶן. וְיֵשׁ בַּעַל נֶפֶשׁ רְחָבָה שֶׁלֹּא תִּשְׂבַּע נַפְשׁוֹ מִכָּל מָמוֹן הָעוֹלָם, כָּעִנְיָן שֶׁנֶּאֱמַר (קהלת ה ט) "אוֹהֵב כֶּסֶף לֹא יִשְׂבַּע כֶּסֶף". וְיֵשׁ מְקַצֵּר נַפְשׁוֹ שֶׁדַּיּוֹ אֲפִלּוּ דָּבָר מְעַט שֶׁלֹּא יַסְפִּיק לוֹ וְלֹא יִרְדֹּף לְהַשִּׂיג כָּל צָרְכּוֹ. וְיֵשׁ שֶׁהוּא מְסַגֵּף עַצְמוֹ בְּרָעָב וְקוֹבֵץ עַל יָדוֹ וְאֵינוֹ אוֹכֵל פְּרוּטָה מִשֶּׁלּוֹ אֶלָּא בְּצַעַר גָּדוֹל. וְיֵשׁ שֶׁהוּא מְאַבֵּד כָּל מָמוֹנוֹ בְּיָדוֹ לְדַעְתּוֹ. וְעַל דְּרָכִים אֵלּוּ שְׁאָר כָּל הַדֵּעוֹת כְּגוֹן מְהוֹלֵל וְאוֹנֵן וְכִילַי וְשׁוֹעַ וְאַכְזָרִי וְרַחֲמָן וְרַךְ לֵבָב וְאַמִּיץ לֵב וְכַיּוֹצֵא בָּהֶן: וְיֵשׁ בֵּין כָּל דֵּעָה וְדֵעָה הָרְחוֹקָה מִמֶּנָּה בַּקָּצֶה הָאַחֵר דֵּעוֹת בֵּינוֹנִיּוֹת זוֹ רְחוֹקָה מִזּוֹ. וְכָל הַדֵּעוֹת יֵשׁ מֵהֶן דֵּעוֹת שֶׁהֵן לָאָדָם מִתְּחִלַּת בְּרִיָּתוֹ לְפִי טֶבַע גּוּפוֹ. וְיֵשׁ מֵהֶן דֵּעוֹת שֶׁטִּבְעוֹ שֶׁל אָדָם זֶה מְכֻוָּן וְעָתִיד לְקַבֵּל אוֹתָם בִּמְהֵרָה יוֹתֵר מִשְּׁאָר הַדֵּעוֹת. וְיֵשׁ מֵהֶן שֶׁאֵינָן לָאָדָם מִתְּחִלַּת בְּרִיָּתוֹ אֶלָּא לָמַד אוֹתָם מֵאֲחֵרִים אוֹ שֶׁנִּפְנָה לָהֶן מֵעַצְמוֹ לְפִי מַחֲשָׁבָה שֶׁעָלְתָה בְּלִבּוֹ. אוֹ שֶׁשָּׁמַע שֶׁזּוֹ הַדֵּעָה טוֹבָה לוֹ וּבָהּ רָאוּי לֵילֵךְ וְהִנְהִיג עַצְמוֹ בָּהּ עַד שֶׁנִּקְבְּעָה בְּלִבּוֹ:
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.