Good question -- something that a lot of people worry about. Tolstoy famously fell into a deep depression about this very question. He ultimately decided that the only way to find meaning in life was through faith, and he embraced God. But many philosophers have disagreed that this is the only route to meaning, and they have argued that meaning in life in possible even if God does not exist. In part this depends on what we take "meaning" to be. You put it in terms of "purpose", but tht might not be the only way to think about it. Philosophers like Richard Taylor have argued that we can find meaning by seeking inner fulfillment -- living a fulfilled life is to live a meaningful life. Alternatively, other philosophers like Peter Singer have argued that we can find meaning in life by adopting objectively worthwhile projects, ethical projects. By doing good -- be it working to end world hunger, or curing diseases, or fighting injustice -- we can infuse our lives with meaning. And yet other...

Beware of straightforward answers to questions -- otherwise, you may end up like the folks who built the computer Deep Thought to answer the question of Life, the Universe, and Everything. When Deep Thought delivered the answer ("42") after 7.5 million years, it was a very straightforward one. But in this case it turned out that to understand the answer, you have to understand the question -- and no one really did. Getting clear on what the truly important questions are -- and how they should be understood -- is thus critical if we are to have any hope at answers that we can make sense of. So the question-asking role of philosophy is not one to be lightly dismissed. That said, I don't think it's true that philosophy fails to provide answers. You might be interested in Gary Gutting's recent book, What Philosophers Know , which attempts to show some answers that philosophy has provided in recent years.

I agree with the posts above on the decisiveness of Hume's criticisms of the teleological argument in the Dialogues Concerning Natural Religion , but thought I'd add one point on the other side. In The Blind Watchmaker , Richard Dawkins suggests that he "could not imagine being an atheist" before Darwin's Origin of Species was published in 1859. Dawkins suggests that "although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist." Dawkins argues that "what Hume did was criticize the logic of using apparent design in nature as positive evidence for the existence of a God. He did not offer an alternative explanation for apparent design, but left the question open." It was only with Darwin that a plausible alternative explanation was provided.

According to David Hume in his essay "Of Miracles," a miracle "is a violation of the laws of nature." Hume goes on, by the way, to argue that it is never rational for us to believe that a miracle has occurred.