A baby is God’s opinion that life should go on.

Carl Sandburg (1878-1967) American poet, biographer

Remembrance Rock, ch. 2 (1948)
    (Source)

Orville Brand "Bowbong" Windom speaking to his grandson, Raymond. Sometimes misquoted as "A baby is God’s opinion that the world should go on."

While popular in its own right, the broader context of the quotation is also worth noting. Windom is recounting a story of a man criticized for sleeping through a play he was supposed to be reviewing, who said, "Sleep is an opinion." Windom continues:

And a baby is God's opinion that life should go on. A book that does nothing to you is dead. A baby, whether it does anything to you, represents life. If a bad fire should break out in this house and I had my choice of saving the library or the babies, I would save what is alive. Never will a time come when the most marvelous recent invention is as marvelous as a newborn baby. The finest of our precision watches, the most super-colossal of our supercargo planes, don’t compare with a newborn baby in the number and ingenuity of coils and springs, in the flow and change of chemical solutions, in timing devices and interrelated parts that are irreplaceable. A baby is very modern. Yet it is also the oldest of the ancients. A baby doesn’t know he is a hoary and venerable antique -- but he is. Before man learned how to make an alphabet, how to make a wheel, how to make a fire, he knew how to make a baby -- with the great help of woman, and his God and Maker.