"The overriding philosophy in Episode I—and in all the Star Wars movies, for that matter—is the balance between good and evil."
-George Lucas, quoted in L. Bouzereau, Star Wars: The Making of Episode I, 1999
"In each of us we to have balance these emotions, and in the Star Wars saga the most important point is balance, balance between everything."
-George Lucas, Time Magazine article, 2002
"The idea of positive and negative, that there are two sides to an entity, a push and a pull, a yin and a yang, and the struggle between the two sides are issues of nature that I wanted to include in the film."
-George Lucas, quoted in L. Bouzereau, Star Wars: The Annotated Screenplays
"The Force has two sides. It is not a malevolent or a benevolent thing. It has a bad side to it, involving hate and fear, and it has a good side, involving love, charity, fairness and hope."
-George Lucas, Times Magazine, 1980
"I wanted to have this mythological footing because I was basing the films on the idea that the Force has two sides, the good side, the evil side, and they both need to be there. Most religions are built on that, whether it's called yin and yang, God and the devil—everything is built on the push-pull tension created by two sides of the equation. Right from the very beginning, that was the key issue in 'Star Wars.'"
-George Lucas, Times Magazine, 2002
"The Light is positive. It is intimately bound with the essence of living things; it is peace, harmony and knowledge. The Light Side springs from the great pattern of existence. It draws strength from diversity and tolerance. It is also inherently communal in nature, thriving on cooperation. Those emotions that enhance the existence of the whole flow from it and tap into its great reserves of strength and peace. Patience, humility and self-sacrifice are paths to enlightenment. Above all, it seeks harmony and perfection.
The Dark Side, in comparison, is the force of entropy and destruction. Chaos and rage feed it and are its sources of power. The Dark Side is a part of nature — it is not inherently evil, but evil comes from its irrationality, its intolerance and its lack of control. ******* and predatory, domination is its goal. Mercilessly aggressive and unforgiving, its adherents are blinded by greed and lust for power over those weaker than themselves.
The Light and Dark Side manifest themselves in the way they are used; they are simply different interpretations of a single aspect of nature, and they exist in balance with themselves and the universe. Just as with any aspect of life and death, both the Dark Side and the Light Side are intertwined with each other, are necessary to each other and form a cosmic balance."
-The Dark Empire Sourcebook
And Mortis clarifies this theme again.