A few people have asked me recently what I think the purpose or meaning of life is as an atheist. They would be correct to say that atheism itself cannot inform me about the meaning or purpose of life, since atheism is defined as merely being a lack of belief in any gods (it should be pointed out that mere theism -- that is, the mere belief in a god or gods -- cannot inform someone about the meaning or purpose of life either. Only particular theistic doctrines or philosophies can inform on that level.) Nonetheless, I do have an answer that I believe in with regards to this question, and I would happily share it.
First, a few comments about the word "purpose". Usually, we describe artifacts, inventions, or tools as having purposes because they were constructed by intelligent beings for some use. For example, we describe the purpose of hammers as being to insert nails into wood, the purpose of pipes to channel water, the purpose of cars being transportation, and so on. Presumably, this is what members of Western monotheistic religions have in mind when they talk about human lives having purpose; i.e. there is an intelligent being (namely, God) who constructed humans for some use, and that human beings should live out their lives in accordance with that divine plan.
Personally, I actually find this notion disturbing -- I do not find it comforting to think of myself as being used in some cosmic way, as if I am merely some kind of tool. I find it intensely liberating to think of myself as being an individual who is not constrained by divine mandate. Of course, as I am fond of reminding people, what we wish to be so isn't always so. Thus, I am not presenting here an argument against God so much as a presentation of my own personal feelings in the hope that the reader can find something to empathize with.
Next, I want to make a few comments about the word "meaning". The word "meaning" usually describes one of two things -- either (1) the semantic content of a word (or a set of words conjoined into sentences, paragraphs, or other linguistic structures) or (2) the importance of something, usually expressed with some level of elation or intense emotional experience. The former use of the word "meaning" is that used when I say something like "the word 'cat' means a certain kind of small furry carnivorous mammal known to scientists as Felis Catus." The latter sense is that used when I say something like "my relationship with this girl means a lot to me; I'd be willing to sacrifice a lot for it."
Quite obviously, the word "life" has meaning in the former sense. Life (not the word, but the thing we are living) does not have meaning in the former sense and to say that it does would be to make a severe use-mention error. Life can no more have meaning than cheeseburgers (the food item) contains four e's. If this is the meaning that people attach to the phrase "meaning of life" then we can rightfully state that this is what Daniel Dennett identifies as a deepity.
Nonetheless, I think a more charitable interpretation of the phrase "meaning of life" would be referring to some innate importance of life. But this is troublesome to me too, because, as far as I can tell, importance is relative to the person. It might be that one person thinks life is important and another does not. I have in mind a certain objection about God being able to confer absolute (and therefore innate) importance onto whatever He like. However, provided that God exists, I claim that while He could think life is important, it would still fail to be the case that life would have innate importance.
Why? Because it does not follow from God conferring importance to x that x has innate importance. For example, events occurring in the Andromeda galaxy might be very important to God for reasons unbeknownst to us. However, those events might not be important at all for human beings (i.e. they are divine concerns, not human concerns.) In that sense, whatever importance that God confers onto x does not imply that humans are necessitated to confer importance onto x. A key example of this arises in the Christian doctrine that only God is allowed to judge human beings. If only God is allowed to judge human beings, then there will be particular issues which are of concern to Him as divine judge which simply fail to arise in our concerns as human beings. Those particular issues would presumably have some hierarchy of importance. By necessity, human beings would not confer the same hierarchy of importance unto those issues since they fail to stand as divine judges.
Frankly, I can't honestly make good sense out of the phrase "meaning of life". Whenever I ask religious people about this phrase, they give an answer that has nothing to do with the various definitions of the word "meaning" that I am familiar with. For that reason, I will leave the word "meaning" behind and instead focus specifically on the word "purpose".
I have already stated that since I lack belief in the existence of any supernatural beings who purportedly created human beings for some use, I do not believe that we have purpose in that sense. However, there is another sense in which we can have purpose.
Those things which have purposes are those which have some use for effecting some goal. As we live our lives, we have goals. Whether those are temporary goals or long term goals, we have various feats that we would each like to accomplish. As such, we give our lives purpose by using our lives to effect those goals as best as we can. Seizing the day, we should live our lives to the fullest, cherishing every possible moment.
As someone who does not believe in an afterlife, I believe that we are each lucky enough for every moment that we are alive. As Richard Dawkins has pointed out, there are vastly more human beings that could have lived than presently do or ever will. Only a few lucky individuals get to rise out of the void before sinking back into it. We are the lucky ones, those who actually get to live for some short time.
Since our lives are finite, I cherish every moment that I have with those that I love (this is not to say that those who believe in an afterlife cherish their moments with their loved ones any less, but this is how my perspective on the world informs me.) The moments that I have in conversation with my parents or with long time friends and those who I am intimate with -- these are the moments that I treasure as I pass through them. I call my parents frequently partly in the recognition that they will not be around forever. And I keep the memory of those who have past away.
Since my time is finite and since I am a human being equipped with a conscience, I endeavor not to make moral transgressions or wreak havoc upon the Earth. I strive to strengthen my ties with my fellow human beings, and to make a positive impact on those around me. I do this not to appease any deities and not in accordance with any holy book, but merely to be good for its own sake. It deeply disturbs me when some religious people tell me that if God did not exist, they would be out raping and pillaging without remorse.
I value science and rational thought as my guides to the operation of the world. I see them not as cold reservoirs of sterile knowledge, but beautiful tapestries of all too human (and therefore feeble) interactions and explorations of the natural order. I see profound beauty in not just in those things readily observed, like starlit nights or the cavernous bowels of the grand canyon, but also in the microcosm of cellular matter. And deeper still is more profound beauty, in the sublime yet ever confusing quantum mechanical world. On the other end of the cosmos, I see solar systems, galaxies, and galactic structures, held together by the ancient primordial force of gravitation -- a simple mathematical rule expressed in the clockwork actions of the heavens. While I do not think that reductionism can ultimately explain absolutely everything, I do not see anything ugly in it either, each scale in the reduction having heart aching allure. Richard Feynman once expressed this sentiment in a way that moves me:
"I have a friend who’s an artist and he’s some times taken a view which I don’t agree with very well. He’ll hold up a flower and say, 'look how beautiful it is,' and I’ll agree, I think. And he says, 'you see, I as an artist can see how beautiful this is, but you as a scientist, oh, take this all apart and it becomes a dull thing.' And I think he’s kind of nutty. First of all, the beauty that he sees is available to other people and to me, too, I believe, although I might not be quite as refined aesthetically as he is. But I can appreciate the beauty of a flower. At the same time, I see much more about the flower that he sees. I could imagine the cells in there, the complicated actions inside which also have a beauty. I mean, it’s not just beauty at this dimension of one centimeter: there is also beauty at a smaller dimension, the inner structure… also the processes. The fact that the colors in the flower are evolved in order to attract insects to pollinate it is interesting — it means that insects can see the color. It adds a question — does this aesthetic sense also exist in the lower forms that are… why is it aesthetic, all kinds of interesting questions which a science knowledge only adds to the excitement and mystery and the awe of a flower. It only adds. I don’t understand how it subtracts."
I bask in the supreme glory of the universe around us, and I unabashedly use religious metaphors in describing it. I would not go so far as to describe my efforts to educate the public about science as proselytizing. Nonetheless, I stand firm with Neil DeGrasse Tyson when he describes the stellar origins of the matter which atomically composes human beings as a statement so epically profound it makes him want to shake people on the street and ask "have you heard this?"
I even see Orphic charm in mathematics, perhaps more so there than in anything else. This sentiment was probably best described by Bertrand Russell when he stated, "Mathematics, rightly viewed, possesses not only truth, but supreme beauty -- a beauty cold and austere, like that of sculpture."
I don't know why the poets have been so silent in describing science and mathematics. Perhaps far too many of them see the subject as being soulless, and unable to describe or capture the human condition. That I think this is naive is a cyclopean understatement.