This is as opposed to the fine tuning arguments, which has a bunch of really big, entirely made up, arbitrary numbers meant to look impressive and convincing. The odds the strong nuclear force would have a value capable of allowing star formation is 1 in 800,000? Says who?
I wouldn't think it necessary to explain why we know that the universal constants must have been fined tuned for life to exist. After all, even Richard Dawkins acknowledges that much to be true. But if you dispute the point, I've already mentioned two books that explain the reasoning at length:
The Mind of God by Paul Davies and
Just Six Numbers by Martin Reese. A few excerpts from the first one:
[FONT="]We can write down the equations of physics and then tinker with them a bit to see what difference it makes. In this way theorists can construct artificial-model universes to test mathematically whether they can support life. Considerable effort has gone into studying this question. Most investigators conclude that the existence of complex systems, especially biological systems, is remarkably sensitive to the form of the laws of physics, and that in some cases the most minute changes to the laws are sufficient to wreck the chances of life arising.
[/FONT][FONT="]Most scientists have tacitly assumed that an approximately non-quantum (or "classical", to use the jargon) world would have emerged automatically from the big bang, even from a big bang in which quantum effects dominated. Recently, however, Hartle and Gellmann have challenged this assumption. They argue that the existence of an approximately classical world, in which well-defined material objects exist at distinct locations in space, and in which there is a well-defined concept of time, requires special cosmic initial conditions. Their calculations indicate that, for the majority of initial states, a generally classical world would not emerge.[/FONT]
[FONT="]A careful study suggests that the laws of the universe are remarkably felicitous for the emergence of richness and variety. In the case of living organisms, their existence seems to depend on a number of fortuitous coincidences that some scientists and philosophers have hailed as nothing short of astonishing.[/FONT]
Well, he's got some probability to justify his arguments. It's not like he can say "The probability of a life-bearing universe is X." But given 4.2 kerjillion randomly generated universes the probability that at least one contains life is much higher than if there's 4. It's a pretty simple concept.
That ignores a problem that I've already pointed out. Even if vast numbers of universes were springing out of nowhere, there's no reason to believe that each one would have a slightly different set of physical parameters: slightly different total mass, slightly different total energy, slightly different gravitational constant, etc... Even if we knew of some mechanism that causes universes to spring out of nowhere--which, needless to say, we do not--a mechanism which produced many universes with the same properties seems much more probable than one in which the properties all vary a tiny little bit and thus allow one to escape the fine-tuning argument via multiple universes.
Then there are the two other objections that neither Dawkins nor any of his fans can answer. First, they're constantly telling me not to believe in things when there's no evidence for those things. There's certainly no evidence for any universe other than this one. No one's ever seen, heard, smelled, tasted or touched such a universe, nor any evidence originating from or otherwise suggesting the existence of such a universe. So, to be consistent with how atheists tell me to think, I have to reject any claims about multiple universes.
Second, a vast multiverse is much more complex (by any definition) than a God capable of creating one universe. So by Dawkins' own reasoning the later is preferable to the former as an explanation for the existence of a universe capable of supporting life.
it ignores the anthropic principle.
Like many people I find the anthropic principle utterly unsatisfactory; in fact, so does Dawkins. The fact of the matter is that we have a universe capable of supporting life; here we are, as proof of that. So it's natural for as to ask the reason for it. The anthropic principle doesn't give a reason.