The fine tuning argument is one of my pet hates. It has left me astonished on numerous occasions. I am not astonished by its brilliance or by how difficult it is to refute as many religious folk might presume. I am astonished by how much weight sensible, intelligent atheists give to it. It is a deeply flawed argument. Let’s ignore for a moment the obvious implications of the anthropic principle. Let’s just take a look at the argument. It proposes that the fundamental constants of the universe, for instance gravity, are set exactly where they need to be in order for life to exist. An example might be something like the nuclear forces. Let’s assume they are extremely weak. If this were the case, it would be impossible for elements to form (except perhaps in some very specific cases). The matter in the universe would just be clumps of fundamental particles clustered in gravity wells. See! Life would be impossible! If you follow this line of reasoning, the fine tuning argument need not be directed towards life at all. You would have just as strong a case (stronger even) for the claim that the universe is fine tuned for the existence of heavy elements. Long live uranium!!!!
It presupposes a special privileged status for life. It essentially begs the question. The assumption that it must be all about life is inherent in the argument.
Forget all that for now though. There is a much more fundamental problem with this argument.
It uses our current physical models of this universe to predict the results of changing some of the constants of the physical laws. That is ok to a point. We can predict what our universe would be like if gravity was much stronger or weaker (to a point). It is a reality of science that experimentation is necessary in order to discover those things you didn't consider. The knock-on effects of changing even one of the constants of the universe would be near impossible to predict as none of these constants operate in isolation from each other. Usually, we are told that if x were different, then y would cause this or that. No consideration is given to how w, u or z might also be affected and what effect those affects would have on all the other constants and what effect those affects would have etc. (How many of you just spent a good few seconds judging my use of the words effect and affect? - flaming, nit-picking atheists :P. For the record, I gave no consideration to which was warranted in any case there!) To make matters far worse, usually only one of the constants is chosen to be altered at any given time. No consideration is given to the massive number of possible universes in which many or even all constants are changed at the same time. The resultant universe would be simply too different to accurately assess. What kind of life might exist in such universes? No idea, probably nothing like us, but would life of any kind be possible. No-one can say.
Consider the following analogy.
Let’s assume that humans are the only form of life in the universe. A fine tuning proponent attempts to argue that human organs are fine tuned to allow for a living body and that if you changed any of the constants of human anatomy, a living body would be impossible.
"Consider the heart!. Were it much smaller, it would be unable to pump the blood with sufficient pressure to provide adequate circulation. The organs would be starved of oxygen and sustenance. Death would be assured. Were it much bigger, it would pump the blood with more force than the arterial walls could safely endure. Haemorrhage would be inevitable!
That is just the heart. The lungs too must operate within precise parameters. Too much mucus for example and you would literally drown, too little and debris would collect in the lungs and you would suffocate. There are literally thousands of parameters within the body that must be just so to allow the body to live. The only logical conclusion is that some kind of intelligence designed the human body to allow it to live!!......... Hail Jesus!" - I am not a doctor, a biologist nor educated in any way in human anatomy. I am sure there are much better ways biologically speaking to make the point above. I am strictly using this analogy to illustrate a point!
This individual might be taken seriously in a world were there were no other forms of life. Even if other forms of life relatively similar to human did exist, their commonalities may be presumed to be fundamentally necessary. Assume dogs, horses and pigs also existed. What do you think are the chances that these people, living in a world that only consists of humans, dogs, horses and pigs would be able to predict or even imagine a very different form of viable life like an earthworm or a jellyfish?
The truth of the matter is that we cannot even begin to imagine what universes might possibly exist under much different conditions than our own. There may be entirely different systems that are possible in which concepts like gravity don't even make sense. To try to make the claim that only this universe could support life is at best arrogant and incredibly presumptuous.
Down with this silly argument I say!