To admit this is to poise oneself on "the knife edge of despair" in Nobel Laureate Steven Weinberg's words, so it's not unusual for naturalists or atheists (the two words are close enough in meaning as to be considered synonyms) to push back.
One typical rejoinder to the claim that life in the absence of God is meaningless was stated by an atheist named Luke who wrote the following to Christian philosopher William Lane Craig:
I believe that purpose to life is not, nor should be cosmologically significant nor intrinsic but something we create and choose for ourselves. Hence, meaning comes from endeavors which could involve pursuing passions, raising a family, or helping others – pursuits which give your life purpose and make it feel as if it is worth living but have a finite and limited impact in any ultimate sense.Luke's letter reminded me of biologist Theodosious Dobzhansky's bleak asseveration that "The only plausible answer to the problem of the meaning of life is to live, to be alive and to leave more life."
You may argue that if we are going to perish, why does it matter, it will not make any difference if I choose a or b. To illustrate why I disagree with this attitude, let’s consider two scenarios:
1. I have just finished my meal at a buffet restaurant and see a chocolate cake on a stand. Me choosing to eat the cake would be inconsequential in any ultimate sense. But the cake is sweet and delicious, eating it makes the moment temporarily more enjoyable, which is good enough. Eating the cake is a worthwhile experience despite the fact that it will run out and will not have any further impact on my life.
2. I am attending a classical concert and know that the music will end, and the event will become a vague memory. But while I am seated in the audience, I enjoy the music which is so powerful, elegant, and enlightening. In this sense, attending the concert was worth it – it was good while it lasted.
So, on naturalism, we will die and perish, the sun will swallow the earth, the universe will cease to exist. But I think it does matter that we were alive.
During the flicker of time while we were here, life was gratifying, engaging, and beautiful. So, under my conception of meaning, life is meaningful under naturalism, the fact that things to come to an end is not and should not be a cause for fear and despair, but rather something I have blissfully accepted. Instead, we should have gratitude that it happened.
At the conclusion of his substantive response to Luke's argument Craig writes, "Let me close with a question for you to think about, Luke: you say that we should be grateful that the universe happened. Grateful to whom?"
Good question.
But Craig's response aside, my own answer to Luke would be this:
For something's existence to be meaningful it must have a purpose, that is, an end or telos or reasons for its existence. Reasons, however, presuppose a mind in which those reasons reside.
If there is no mind behind the reality in which we live, if the universe is a result of a mindless series of purposeless events, and if humanity is likewise the result of a mindless series of accidental evolutionary coincidences, then there's no reason for humanity's existence and thus no meaning to it. And, if the universe's existence has no meaning, and if life in general has no meaning, and if mankind's existence in particular has no meaning it's very hard to see how an individual life could somehow, in the midst of all that meaninglessness, nevertheless have a meaning, no matter how much chocolate cake we eat.
Certainly, while we exist some of us can enjoy life's pleasures, but is pleasure the purpose or reason for our existence? Is enjoyment the whole point of our lives? If so, then the lives of most people who've ever lived have had a very attenuated meaning at best because for most people throughout human history to live is to suffer.
And if pleasure is not the reason for our existence, then pleasure can't be what gives life meaning. It only helps to make life endurable.
So, if we are the accidental product of mindless processes that blindly and purposelessly brought us into being then there's no reason for our existing, no end or purpose for which we are made. We're just dust in the wind and all that we do and enjoy ultimately comes to nothing.
As filmmaker Woody Allen once said, "You have a meal, or you listen to a piece of music, and it's a pleasurable thing, but it doesn't [amount] to anything."
If naturalism is true then the claim of another filmmaker, Ingmar Bergman, is as apt as it is succinct: “You were born for no purpose. Your life has no meaning. When you die you are extinguished.”
On the other hand, if naturalism is false and we're actually the product of an act of intentional creation - if a Mind brought us into being - then we can assume that this Mind had a reason for doing so. We can assume that we have a purpose, an end toward which we are to strive, and that our lives are therefore meaningful.
Even if we don't know the reason for which we were created we can at least assume there is one, and if we were purposely created then our existence is not just a meaningless random accident.
It's a wonderful prospect, but for some perplexing reason naturalists prefer to believe the melancholy view that they're headed for oblivion, that nothing really matters and that all we do is ultimately for naught, rather than embrace the possibility that they were created by a God who loves them and that their lives really do matter. Forever.