The Anthropic Principle (AP) concerns the relationship between the laws of nature and the presence of life. A general definition of the AP is as follows: The laws of the universe are fine-tuned to allow life to exist. If the laws were changed ever so slightly, life would not exist.
Here are some examples.
Example 1. “If the nuclear force were only a few percent weaker, then a proton could not combine with a neutron to form a deuteron. If this were the case, no deuterons would be formed in the sun and hence no solar fuel would exist. As a result, the sun would not shine (‘burn’), but would merely be a cold ball of inert gas—precluding the possibility of life on Earth.” (http://www.simpletoremember.com/articles/a/anthropic-principle)
Example 2. A slightly stronger gravity force would result in “a smaller universe of larger, brighter, shorter-lived stars, that will eventually collapse in on itself again in a Big Crunch. Most, if not all stars would be binary, trinary, or larger systems. Any planets orbiting these stars would have to go very fast to avoid a fiery demise inside their parent stars, and would be slung around wildly by their multiple suns. Any planetary system in this universe would be devoid of a stable, safe harbor for life, and relative stability is a vital prerequisite for the evolution of complex life forms. Life here would probably get to no more than amino acids, much less true life, before one of the planet’s parent stars went nova or the planet was torn apart and swallowed into one of the stars. It would not be a place to develop living creatures as complex as ourselves.” (http://www.physics.sfsu.edu/~lwilliam/sota/anth/coincidence.htm)
There are different forms of the anthropic principle. The one form that most scholars agree upon is the Weak Anthropic Principle (WAP). This principle claims that conditions in the universe are such that they allow life to exist. If this statement did not create a “eureka moment” for you, you’re not alone. We already know that the conditions of the universe are such that they allow for life by virtue of the fact that we exist. Thus while everyone agrees with the WAP, it is a tautology (a necessary and unproductive truth).
Are other anthropic principles more substantive? They are, but they are also more controversial. Here they are.
Strong Anthropic Principle (SAP) - the universe must have the properties it has for life to exist.
Participatory Anthropic Principle (PAP) - the universe needs conscious observers to bring it into existence. (There are philosophical and quantum mechanical reasons for this claim.)
Final Anthropic Principle - because the universe is fine-tuned, intelligent life forms must come into existence, and once they do, they never become non-existent.
The main point of this post is that some believers like to invoke the SAP as evidence for the existence of God. If the universe must have certain properties for life to exist, and we exist, then we may conclude that the universe was specifically designed for the purpose of sustaining life. As a latter-day saint, I wholeheartedly accept the SAP premise, but it lacks rigor. I advise against using it as a rational argument for the existence of God.
The problem with marveling at a fine-tuned universe (and invoking God as the first cause) is that we could not do otherwise. Imagine that there are 10 multiverses (10 multiple universes) in existence. If 5 of those universes are fine-tuned to allow human life, then people in those 5 universes will necessarily marvel over the fact that their universes are fine-tuned. Some of them will even propose a god as the designer. If the other 5 universes are not fine-tuned, then there will be no one to question why their universes are not fine-tuned.
IMO, for an argument to be considered rational, it must win out against less rational opposing arguments. But if just one argument is possible (in this case, concluding that it is amazing that our universe is fine-tuned), then there is no way that the argument can win out over other competing arguments. Without a competing argument, we cannot declare a winner.
Moreover, just because there are no competing arguments, it does not mean that the one argument we have wins by default. Under these circumstances, the argument we have is a tautology – nothing more than a statement that is necessarily true. Tautologies do not carry much weight in scientific and philosophical debates.
In sum, we know that the natural laws in our universe could be different, and we know that if they were different, life would not exist. However, if the laws of nature were different, then we would not exist to ponder these issues. Thus we can entertain just one possibility – that it is amazing that our universe is fine-tuned to support life.
Being limited to just one position does not result in a rational argument. We must be able to acknowledge and refute counterpositions in order to make an argument rational.