One argument for the existence of God is that there are things in nature that are so complicated that they cannot have arisen by purely natural processes and so must have been designed by an intelligent agent (i.e. God). I used to be a Creationist, and this argument resonated with me. I won't talk about the creation/evolution debate right now, that's a subject for a later post.
An counter-argument to the above could be to say that if the things which are designed are complicated, then the being who designed them must be more complicated still, so who designed the designer? This counter-argument never used to persuade me at all - you can't ask "who designed God?" - God just is and that's all there is to it.
However, recently I've come to understand the significance of it a bit more. We are trying to resolve the issue of complexity in nature. There is complexity, there is apparent design, we can see it all around us. Where did it come from? How can we explain it?
There are two competing hypotheses here - a) it all arose through natural, mindless processes, or b) they are the result of intelligent agency. We have to decide which of these hypotheses has the greater explanatory power. The first hypothesis attempts to explain complexity by attributing it to the results of simpler, less complex forces (such as natural selection in biology - an observed and proven process, and gravity in cosmology). The second attempts to explain complexity by attributing it to an intelligent agent, who must necessarily be more complex than the objects he/she/it is designing.
Let's put it another way. How can we explain the existence of intelligent beings? Intelligence is presumably so complex a thing that it can only have been designed by an intelligent agent. Does anyone spot the circular argument here? We haven't explained the existence of intelligent beings at all, because our explanation involves invoking an intelligent being - an even more intelligent being, presumably. We are back where we started.
This however is not a problem for someone who believes in God, as they already have as their starting point an intelligent being, God, who they believe exists for reasons other than the design argument. It's the easiest thing in the world to explain complexity in the world if you already believe there exists a being complex and powerful enough to create it.
Can you see though that to someone who doesn't already believe in God this doesn't explain anything at all, and that someone who's really seeking an explanation for why things are the way they are will not value the design argument very highly?