In a flashback scene in the 1977 film Annie Hall, Woody Allen's character Alvy Singer is a depressed young boy who won't do his homework because, as he explains to his doctor: “The universe is expanding.... Well, the universe is everything, and if it's expanding, someday it will break apart, and that will be the end of everything.” His exasperated mother upbraids the youth: “What has the universe got to do with it?! You're here in Brooklyn. Brooklyn is not expanding!”

Call it “Alvy's Error”: assessing the purpose of something at the wrong level of analysis. The level at which we should assess our actions is the human timescale of days, weeks, months and years—our life span of fourscore plus or minus 10—not the billions of years of the cosmic calendar. It is a mistake made by theologians when arguing that without a source external to our world to vouchsafe morality and meaning, nothing really matters.

One of the most prominent theologians of our time, William Lane Craig, committed Alvy's Error in a 2009 debate at Columbia University with Yale University philosopher Shelly Kagan when he pronounced: “On a naturalistic worldview, everything is ultimately destined to destruction in the heat death of the universe. As the universe expands, it grows colder and colder as its energy is used up. Eventually all the stars will burn out, all matter will collapse into dead stars and black holes, there will be no life, no heat, no light—only the corpses of dead stars and galaxies expanding into endless darkness. In light of that end, it's hard for me to understand how our moral choices have any sort of significance. There's no moral accountability. The universe is neither better nor worse for what we do. Our moral lives become vacuous because they don't have that kind of cosmic significance.”

Kagan properly nailed Craig, referencing the latter's example of godless torturers: “This strikes me as an outrageous thing to suggest. It doesn't really matter? Surely it matters to the torture victims whether they're being tortured. It doesn't require that this make some cosmic difference to the eternal significance of the universe for it to matter whether a human being is tortured. It matters to them, it matters to their family, and it matters to us.”

Craig committed a related mistake when he argued that “without God there are no objective moral values, moral duties or moral accountability” and that “if life ends at the grave, then ultimately it makes no difference whether you live as a Stalin or a Mother Teresa.” Call this “Craig's Categorical Error”: assessing the value of something by the wrong category of criteria. In my new book, recently published, Heavens on Earth, I debunk the common belief that without God and the promise of an afterlife, this life has no morality or meaning. We live in the here and now, not the hereafter, so our actions must be judged according to the criteria of this category, whether or not the category of a God-granted hereafter exists.

Whether you behave like a Soviet dictator who murdered tens of millions of people or a Roman Catholic missionary who tended to the poor matters very much to the victims of totalitarianism and poverty. Why does it matter? Because we are sentient beings designed by evolution to survive and flourish in the teeth of entropy and death. The second law of thermodynamics (entropy) is the first law of life. If you do nothing, entropy will take its course, and you will move toward a higher state of disorder that ends in death. So our most basic purpose in life is to combat entropy by doing something “extropic”—expending energy to survive and flourish. Being kind and helping others has been one successful strategy, and punishing Paleolithic Stalins was another, and from these actions, we evolved morality. In this sense, evolution bestowed on us a moral and purpose-driven life by dint of the laws of nature. We do not need any source higher than that to find meaning or morality.

In the long run, entropy will spell the end of everything in the universe and the universe itself, but we don't live in the long run. We live now. We live in Brooklyn, so doing our homework matters. And so, too, does doing our duty to ourselves, our loved ones, our community, our species and our planet.