The teleological argument for the existence of God covers many things. The watchmaker, the boat builder, the painting and the fine tuning argument are all examples of this argument. They all suffer from similar flaws.
The Watchmaker Argument
If an ancient man were walking down the beach and stumbled upon a watch, he would look at it and say to himself, “This watch is complex, so someone must have designed this.” Similarly, the complexity of man suggests that someone must have designed him. This person must be much more complex than the thing it designed, therefore God must exist.
This argument is similar to the boat builder and the painting argument, and is indistinguishable other than the man-made thing they are talking about.
My watch has a watch maker, therefore God exists.
Fine Tuning
The universe, and earth are so finely tuned that there is no possibility that it could have all come about by chance. If the weak nuclear force were slightly different, there would be no way for the universe to form stars, if gravity were slightly different, the earth would not orbit the way it does and would not support life. Because these things are made the way they are, this means God did it.
If the universe was different, the universe would be different, therefore God exists.
Examination
Well, it is true that if you never saw a watch before and you stumbled upon one, you may insist that it is designed, and arguing any differently would be unsupportable since there is no mechanism that we can imagine that would allow a watch to make itself.
The fine tuning argument I can’t really examine without taking a leap of faith in the first place. You have to assume that the forces of nature are determined in such a way specifically so that life can exist. There is no reason to believe this to be true.
Does complexity require a designer?
Complexity is a subjective thing. Looking at a rock, it doesn’t look very complex. If you pile a bunch of rocks on top of each other randomly, it doesn’t build a house, so you may say that it doesn’t look very complex either. If you examine it though, you can see that a pile of rocks is a perfect home for insects. To insects it is perfectly complex. If you have a single hydrogen atom in space, it is not very complex, but if you pile on enough hydrogen, it can start a star which is infinitely more complex than hydrogen. If you look at a meteoroid, it is not complex, but if you pile enough of them together, you get a planet, and that is much more complex than a meteoroid.
These things are all done by natural forces, and they are all extremely complex depending on your point of view. In the case of man, we can imagine ways in which life can start, and all the elements necessary for that are freely available not only on earth, but in space. All it takes is fortunate chance, and suddenly we have all that is necessary for life to evolve. This watchmaker idea no longer seems so pertinent.
The only problem is when you are arguing with someone who denies that evolution happens. That argument is a different discussion though.
Is the universe fine tuned for our existence?
As I said earlier, the fine tuning argument presupposes that life must exist, and it must be our form of life. There is no evidence that this is the case. As far as we have been able to find out, there is no place in the universe where our form of life is able to exist without technology except for some of the places on our planet. Even on our planet, the majority of the surfaces are inhospitable to human life.
Now I have heard people like William Lane Craig put probabilities on the chances that our universe would have been created the way it is by chance. If it weren’t created the way it was, we simply wouldn’t have been around to ask that question. There is no reason whatsoever to assume the universe is meant for life in general, or us specifically. The fact is, the universe had just as much chance to be created like it was as it had to be created any other way. We are just fortunate that it was created as it was.
Conclusion
We have no reason to believe that our universe was created for us, and we have very good reason to believe that life came from non-life spontaneously. Anyone that would argue differently presupposes things that there is no reason to presuppose.
If someone starts arguing the watch maker argument, let them know that they are arguing a false analogy since we can conceive of no way that mechanical things can make themselves, but we can conceive of methods that life can create itself.
If someone argues the fine tuning argument, ask them why they assume that the universe was created to support life. There is no reason to make such an assumption other than to promote the idea that a God did it, but if there wasn’t a God that did it, there is no reason to jump to the conclusion that this universe came about with the forces we have other than because it did, and probabilities are just silly.