This is from Ethan Hawke’s Rules for a Knight:
One time, on a sweltering August night, Grandfather and I made camp down by the ocean. He said, “While I teach you about the ways of war, I want you to know that the real struggle is between the two wolves that live inside each of us.”
“Two wolves?” I asked, seated on an old log near the fire. My eyes were transfixed by the flames twisting uncomfortably in the night air.
“One wolf is evil,” he continued. “It is anger, envy, greed, arrogance, self-pity, guilt, resentment, inferiority, deceit, false pride.” He paused, poking at the embers of our fire with a long stick he’d been carving.
“The other is good. It is joy, love, hope, serenity, humility, loving-kindness, forgiveness, empathy, generosity, truth, compassion, faith.”
I considered that for a minute, then tentatively asked, “Which wolf will win?”
Sparks danced towards the stars as the old man stared into the glare of the flames and replied, “Whichever one you feed.”
If you like the quote hit this link (Kottke.org) and then the Amazon link in the first paragraph.