The anthropic principle is one of the more remarkable swindles in physics. Indeed it is metaphysics, and that is the essence of the problem for most physicists in accepting it. The anthropic logic is either immensely subtle, by arguing that we, via our mere existence, control the cosmos, or unabashedly naive, by setting aside any physics explanations that any ultimate theory of physics might reasonably be expected to deliver. Metaphysics lacks predictive power, the very core of physics. The anthropic principle is an extreme expression of our ignorance.
In my opinion, the strong anthropic principle has a somewhat dubious character, and it tends to be invoked by theorists whenever they do not have a good enough theory to explain the observed facts (i.e. in theories of particle physics, where the masses of particles are unexplained and it is argued that if they had different values from the ones observed, then life would presumably be impossible, etc.). The weak anthropic principle, on the other hand, seems to me to be unexceptionable, provided that one is very careful about how it is used.
It's trivially true (at least in its less extreme varieties). If you want to reason probabilistically, say, about the universe, you have to make sure you condition everything on the fact that you are there to reason about it. There's a nice example that goes back to Boltzmann. Given a disordered set of particles (call them a mini 'universe') you can write an expression for their entropy. If you pick a 'typical' state it will have high entropy and if you let the system evolve chances are the entropy will stay high. Let the system evolve for long enough and you expect the entropy to dip down low occasionally through chance. Suppose the entropy needs to be very low for life to evolve in the system. Then typically any organism that evolves in this system is going to get a skewed view: they're going to find that they live in a universe with unusually low entropy. Their scientists might spend ages trying to figure out just why they live in a low entropy universe but from an 'observer' outside their universe there's nothing surprising - their universe is a long expanse of extereme boredom with occasional low entropy islands, some of which contain life.
1'. The universe is big—about 15 billion light years in radius.
2'. The expansion of the universe led to a huge number of condensed astronomical objects — at minimum 10 solar systems.
3'. The laws of gravity, nuclear physics, atomic physics, chemistry thermodynamics allow a very diverse set of possible environments, from the frozen cold of interstellar space to the ferocious heat of stellar interiors, with planets, moons, asteroids and comets somewhere in between. Even among planets the diversity is huge—from Mercury to Pluto.
4'. The universe is filled with these diverse environments, most of which are lethal. But the universe is so big, that statistically speaking, it is very likely that one or more habitable planets exists.
I don't think anyone questions these points. But what is it that decides which kind of environment we live in—the temperature, chemistry and so on? In particular what determines the fact that the temperature of our planet is between freezing and boiling? The answer is that nothing does. There are environments with temperatures ranging from almost absolute zero to trillions of degrees. Nothing, determines the nature of our environment—except for the fact that we are here to ask the question! The temperature is between freezing and boiling because life (at least our kind) requires liquid water. That's it. That's all. There is no other explanation. 
This rather pedestrian, common sense logic is sometimes called "The Anthropic Principle." Note that I mean something relatively modest by the A.P. I certainly don't mean that everything about the laws of physics can be determined from the condition that life exists— just those things that turn out to be features of the local environment and are needed to support life.
Let's imagine that the earth was totally cloud bound or that we lived at the bottom of the sea. Some philosopher who didn't like these ideas, might object that our hypotheses 1'—4' are un-falsifiable. He might say that since there is no way to observe these other regions with their hostile environments—not without penetrating the impenetrable veil of clouds—the theory is un-falsifiable. That, according to him, is the worst sin a scientist can commit. He will say, "Science means falsifiability. If a hypothesis can't be proved false it is not science." He might even quote Karl Popper as an authority.
From our perspective we would probably laugh at the poor deluded fellow. The correctness of the idea is obvious and who cares if they can falsify it.
Even worse, he wouldn't even be correct about the falsifiability. Here is a way that the anthropic reasoning might be proved false without penetrating the veil of clouds: Suppose an incredibly accurate measurement of the average temperature of the earth gave the answer (in centigrade) T=50.0000000000000000000000000000
degrees. In other words the temperature was found to be exactly midway between freezing and boiling, to an accuracy of one hundred decimal places. I think we would be justified in thinking that there is something beyond the anthropic principle at work. There is no reason, based on the existence of life, for the temperature to be so symmetrically located between boiling and freezing. So discovering such a temperature would pretty convincingly mean that the existence of life is not the real reason why the temperature is between 0 and 100 degrees.
So discovering such a temperature would pretty convincingly mean that the existence of life is not the real reason why the temperature is between 0 and 100 degrees.
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