This time of year, if you notice a house with candles burning in the window, chances are that it’s a Jewish family celebrating Chanukah, the Festival of Lights. The Chanukah candles shine their radiance into the street. Our task is to bring light, morality and holiness not only into our own homes, but also outward into the world.

But there are so many problems out there: terror, environmental damage, natural disasters, countries and continents afflicted by poverty and disease. Can our candles help? The impact we can make feels inadequate against the sheer scale of these tragedies. There are 6 billion people on earth. What difference can one person make? We are no more than a wave in the sea of humanity, sand on the surface of infinity.

This Chanukah has something simple but quite significant to say. We repair the world in small steps, light by light, act by act, day by day. A youth was picking up starfish stranded by the retreating tide and throwing them back into the sea to save them. A man went up to him and said, “This beach goes on for miles, and there are thousands of starfish. Your efforts are futile, it doesn’t make a difference!” The boy looked at the starfish in his hand and threw it into the water. “To this one,” he said, “it makes all the difference.”

That story captures a fundamental idea in Jewish thought. We can’t fix the world all at once. We do it one day at a time, one person at a time, one deed at a time. A single life, say our sages, is like a world. Save a life and you save a world. Change a life and you begin to change the world.

We call this tikkun olam, perfecting the world. Judaism believes that it is no accident that we are here, at this time and place, with these gifts and capabilities, and the opportunity to make a difference. This belief is known as divine providence: the idea that God is active in our lives as individuals, not only, as the Greek philosophers believed, concerned with universals. We are here because there is a task that only we can fulfill. We can never know the ripple of consequences set in motion by the slightest act.

One day a poor Scottish farmer, Fleming, heard a cry for help from a nearby swamp. There, caught up to his waist in black muck, was a terrified boy, screaming and struggling to free himself. Farmer Fleming saved the youth from a slow and painful death.

The next day, a fancy carriage pulled up to the Scotsman’s modest home. A rich nobleman stepped out and introduced himself as the father of the boy.

“I want to repay you,” said the nobleman. “You saved my son’s life.”

“No, I can’t accept payment for what I did,” the Scottish farmer replied, waving off the offer.

At that moment, the farmer’s own son came to the door.

“Is that your son?” the nobleman asked.

“Yes,” the farmer replied proudly.

“Please let me provide him with my own son’s level of education. If the lad is anything like his father, he’ll surely grow to be a man we will both be proud of.”

Farmer Fleming’s son attended the best schools, graduated from medical school in London and became known as Sir Alexander Fleming, who discovered penicillin.

Years later, the nobleman’s son was stricken with pneumonia. What saved his life? Penicillin. The name of the nobleman? Lord Randolph Churchill. His son’s name? Sir Winston Churchill.

Our acts make a difference, sometimes all the difference in the world. Maimonides, one of the greatest sages of the Middle Ages, makes a remarkable statement in the midst of his presentation of the laws of repentance:

“Everyone should regard himself and the whole world as evenly poised between good and guilt. If he commits a sin, he tilts the balance of his fate and that of the world to guilt, causing destruction. If he performs a good deed, he shifts the balance of his fate and that of the world to good, bringing salvation and deliverance.”

One act, says Maimonides, can change a life, and transform a world. How so? Our acts trigger a chain of consequences — psychological, spiritual and historical — that reverberate in incalculable ways. Could Fleming have known that this would change his son’s life, and that his discovery of penicillin would save so many others? Could Fleming have known the rescued child would one day stand alone to save the world from fascism? Obviously not. He could not have known it because the human future is inherently unknowable.

But this we know: We are here, now, in this place, among these people, in these circumstances, so that we can do the act or say the word that will light a candle of hope and holiness in a dark world. “A little light,” said the Jewish mystics, “drives away much darkness.”

And when light is joined to light, mine to yours and yours to others, the dance of flames, each so small, yet so beautiful together, begins to bathe the world in the glow of the Divine presence.

Rabbi Dov Greenberg is the spiritual leader of Chabad at Stanford University.