I've spent a fair bit of time in the last week reading and writing about Atheism as it relates to Nihilism. It strikes me that there are a great many Atheists, but very few Nihilists. Or at least, the Nihilists out there have a much less visible share of the Atheism blog market (which I suppose we should expect from the Nihilist). This has convinced me that it would be worthwhile to make my best argument for Non-Nihilistic Atheism, as this seems to be the path of most who reject the supernatural.
So let us suppose, for a moment, that God does not exist. Let us suppose that we are those infintessimally small cogs in the great machine of the universe, and that we are simply complex chemical reactions bound by the laws of cause and effect. Does it follow that life is inherently meaningless and without intrinsic value?
Well if your definition of "meaningful" is "eternal", then yes, I suppose it does. But if your definition of "meaningful" is "worthwhile", then absolutely not. Regardless of what constitutes a human being (whether it is purely physical or something more) the fact exists that we believe and act as if we are autonomous moral agents. We have this sense of self. The reality of metaphysical principles has no bearing on this. The value of life is in our perception of life- the very fact that we are conscious and aware lends us import. Consider the dog: not many Theists claim that the dog has a "soul". They don't claim he is eternal, anything greater than exactly the physical dog he appears to be. But likewise, we don't consider him without value. We recognize each dog has a personality all its own, and we recognize their value in this individuality; the fact that the dog has a personality is the reason we value him over a computer program.
But if we're to talk in terms of "value" and "meaning", we must eventually tackle morality. Many would claim some objective moral standard is necessary to lend our lives- or at least, our actions- meaning. But this is not the case. Why does utilitarianism suffer in this paradigm? If we all are compelled to do what is best for each other, we can greatly increase all of our enjoyment. Human meaning is not a zero sum game, as the Nihilists would have you believe- everyone out for themselves actually produces a much worse world than everyone out for each other. And since our experience of value is unaffected by whether or not we actually have value, surely utilitarianism it is categorically better than selfishness. You Nihilists can't play the prisoners dilemma on us if we all recognize what you're doing.
We must also deal with the question of free will. Some would claim that Determinism is a defeater of meaning- that is, if everything that will happen must happen, and cannot conceivably be changed, no meaning is possible. After all, how can we claim meaning if we can't even control our own thoughts and actions? But that's a bit like complaining that the characters in a movie did the same thing after you rewound and played it again. Well of course they did. That's what that character would do, by his very nature- else he would be an entirely different character. It is no condemnation of he that takes action that he would have taken the same action presented with the same circumstances once again. And it takes nothing away from the experience of watching the movie that it was filmed and burned to disk long, long before you ever watched it. Since the future is patently unknown and unknowable, determinism is at best 20/20 hindsight. So we can explain why things happen after the fact- so what? That takes nothing away from the reality of the present, just as knowing the dice rolls after a game of risk takes nothing away from enjoying the experience of playing.
It is better, though, to think of this another way. Rather than asking what the implications of a purely physical reality would be, and accepting or rejecting that reality based on these implications, instead think of the idea as already decided. Either there is more than the physical world or there is not, and no amount of wishing or self-deceit can change it. There is nothing more; now what will you do with it? Certainly Nihilism is one approach- but just as certain, there are others. Clearly beauty still exists, even if there is no metaphysical principle underpinning our perception of it ("beauty is in the eye of the beholder", after all). Clearly love is still a powerful and glorious thing. Clearly the common good, basic rights, and the value of human life are as apparent to us as ever. What we have in Physicalism is an excuse to deny them if we wish- not a mandate to do so. There is no reason that says we must live as if these are merely evolutionary fabrications. Our day-to-day lives need be no different than they ever were before; what had value then has value now, if only in that our metaphysical beliefs do not change our underlying perceptions in the slightest
If what you're asking for is a purely rational defense of why we should act in a certain way, I can't give you one. But niether can the one who claims the supernatural as his source. Both of us have given a little bit in rationality to get back something much greater. In the supernaturalist's case, he's gotten back conviction of a higher purpose. In my case, I've gotten back a value that we consistently (even if arbitrarily) attribute to humanity and human causes. A robotic "intelligence" would surely arrive at Nihilism. But a self-aware being has a great many other options.
I must confess that after writing this post, I can see how this is a tennable world view. It sort of has the ring of satisfaction offered by Theism, but allows you to hold on to a bit more of your pure rationality. But ultimately, I still can't buy atheism leading to anything other than Nihilism. Even the arguments I've presented here can be best described as a dressed-up version of Hedonism. My arguments for group morality and meaning are really made out of selfishness- I want the group to be moral because it benefits me. I want to love another person because of the benefits I get from it. There is no basis for altruism here, no basis for self-sacrifice. Hedonism is applied Nihilism in the same way that Physics is applied Math. We can talk about utilitarianism all we want, but ultimately all we're trying to do is maximize our pleasure and minimize our pain. And if the Pleasure/Pain +/- is really trying to maximize the rate at which one set of neural pathways get exercised and minimize the rate of another, then we can't claim any more meaning than a star burning itself out in a glorious explosion, or lighting taking a beautifully twisted path to get to the ground. All of us are simply following the path of least resistance.