The Gemara quotes the famous verse said prophetically about Yaakov and Esav, (Bereishis 25:23):

וַיֹּ֨אמֶר יְהוָ֜ה לָ֗הּ שְׁנֵ֤י גיים [גוֹיִם֙] בְּבִטְנֵ֔ךְ וּשְׁנֵ֣י לְאֻמִּ֔ים מִמֵּעַ֖יִךְ יִפָּרֵ֑דוּ וּלְאֹם֙ מִלְאֹ֣ם יֶֽאֱמָ֔ץ וְרַ֖ב יַעֲבֹ֥ד צָעִֽיר׃

and the LORD answered her,  “Two nations are in your womb,  Two separate peoples shall issue from your body;  One people shall be mightier than the other,  And the older shall serve the younger.”

This concept of one force being dominant and the other weaker, is not merely an accident, nor is it limited to Yaakov and Esau.  There is a fundamental aspect of psychological and material existence that is constantly in the process of moving from one pole to another and then back.

The Greek philosopher Heraclitus (who lived in the time of Ezra but probably never met him) is credited with being one of the first to make this observation, which has been named Enantiodromia.  He observed, "Cold things warm, warm things cool, wet things dry and parched things get wet.” 

The psychologist Carl Jung was the first to discuss the implications of Enantiodromia in human emotional process. Jung understood this as the regulative function of opposites, that is everything tends toward its opposite.  In the repression of psychological urges, the urges or sentiment might flip to an opposite extreme that can be just as bad in its own way. This pattern in life and possibly the Universe is recognized in one way or another by great philosophers and religious thinkers throughout history. This principle was also understood in I Ching, that yang become yin when they have reached their extreme, and the reverse.  

The grand plan on which the unconscious life of the psyche is constructed is so inaccessible to our understanding that we can never know what evil may not be necessary in order to produce good by enantiodromia, and what good may very possibly lead to evil. ("The Phenomenology of the Spirit in Fairytales", Collected Works 9i, par. 397)

Enantiodromia also refers to the process whereby one seeks out and embraces an opposing quality from within, internalizing it in a way that results in individual wholeness. This process is the crux of Jung's notion called the "path of individuation." One must incorporate an opposing archetype into their psyche to obtain a state of internal 'completion.' https://en.wikipedia.org/wiki/Enantiodromia

The Zohar has what to say about this as well

Zohar I:140b

(בראשית כח) וַיֹּאמֶר ה' לָהּ שְׁנִי גוֹיִם בְּבִטְנֵךְ וּשְׁנֵי לְאֻמִּים וְגו'. אֵלּוּ הַשְּׁנֵי גֵאִים הַכָּבֵד וְהַלֵּב. רִבִּי יוֹסֵי אָמַר, הַמּוֹחַ וְהַלֵּב. רִבִּי יְהוּדָה אָמַר, הַמּוֹחַ אֵין בִּכְלַל זֶה, מַשְׁמַע דִּכְתִיב בְּבִטְנֵךְ, וְהַמּוֹחַ אֵין בַּבֶּטֶן אֶלָּא בָּרֹאשׁ. וּשְׁנֵי לְאוּמִּים מִמֵּעַיִךְ וְגו', וְרַב יַעֲבוֹד צָּעִיר זֶהוּ הַכָּבֵד שֶׁהוּא רַב וְגָדוֹל, וְהוּא מְשַׁמֵּשׁ לִפְנֵי הַלֵּב. דְּאָמַר רִבִּי יְהוּדָה, הַכָּבֵד קוֹלֵט הַדָּם וּמְשַׁמֵּשׁ בּוֹ לִפְנֵי הַלֵּב.

"And Hashem said to her, 'Two nations are in your womb, and two peoples'" (Gen. 25:23). These are the two proud ones, the liver and the heart. Rabbi Yosi said that these are the brain and the heart, but Rabbi Yehuda said, The brain is not included in this, for it is written, "in your womb (lit. 'belly')," and the brain is not in the belly, but in the head. "And two peoples from your bowels and the elder (lit. 'great') shall serve the younger." This is the liver, which is great and big, and which serves the heart, as Rabbi Yehuda said, The liver receives the blood, and serves it to the heart!

It seems to me that by identifying the Biblical struggle of Yaakov and Esau as metaphorically the struggles of opposing parts of the body within one person, the Zohar is addressing the concept of psychological wholeness and integration, and the importance of responding to our needs, drives, thoughts and feelings in a balanced manner.