The "anthropic principle" is an attempt to account for our presence in a life-sustaining universe using logic and probability. It ask the question: why are we here, in a universe that supports life, instead of in some other universe? Did God create this universe, or could it have emerged completely by chance?
THE WEAK ANTHROPIC PRINCIPLE (WAP)
The most basic expression of the AP is the "Weak Anthropic Principle," which notes that the reason we can observe that our universe's physical laws are compatible with life is because if they weren't, we wouldn't be here to observe that fact.
Some take this to mean that since life as we know it can only exist within the context of our universe's physical laws, the odds against the appearance of life as we know it must have been very great. This supposed improbability is used to argue that our universe was specially designed and "fine-tuned" by a god or demi-god with a specific interest in human life.
The most obvious objection to the "fine-tuning" interpretation of the WAP is that it assumes that life can only exist in a universe with physical laws exactly like our own; unfortunately, we have no way to verify this assumption, since we can't leave our own universe or even peer out of it.
Even if we assume that our is the only universe, we still have no way of knowing if a different type of universe could have produced life or not.
THE STRONG ANTHROPIC PRINCIPLE
The Strong Anthropic Principle, or SAP, builds on this objection to the "fine-tuning" interpretation of the WAP by stating that we can infer from our own existence that life-sustaining universes are inevitable. The Copernican Principle holds that since our universe exists, and since we have no good reason to think that that it is special, we have to assume that it is simply the most likely of all possible universes - in short, mediocre.
If our own universe is just an average universe, then how is it that it appears to be so exquisitely fine-tuned to permit the existence of intelligent life? One explanation is, of course, that that's just how God made it - the "divine design" argument.
THE ENSEMBLE ARGUMENT
Another is the "ensemble argument", or the idea that our own universe is simply one in an vast ensemble of universes - a multiverse. Many of these universes are inhabited, while others - "dead universes" - possess laws of physics too bizarre or broken to allow for life.
The reason that our own universe appears designed for human life is because we live in a universe which happens to be life-friendly; if we lived in a different type of life-supporting universe, then that too would appear fine-tuned for whatever type of life it supported.
Many different types of ensembles have been proposed. Universes could precede one another in an infinite cycle of big bangs and big crunches, as in the "cyclic universe" scenario:
"In this picture... the Universe undergoes an endless sequence of cycles in which it contracts in a big crunch and re-emerges in an expanding big bang..."
Another possibility is that universes spawn other universes, which in turn "give birth" to other universes, and so on - the "self-reproducing inflationary universe" theory promoted by Stanford's Andrei Linde:
"Recent versions of inflationary theory assert that instead of being an expanding ball of fire the universe is a huge, growing fractal. It consists of many inflating balls that produce new balls, which in turn produce more balls, ad infinitum."
In any event, once we posit an infinite number of universes, probability and improbability fly out the window - not only must there be other life-sustaining universes like this one, but if this one exists, then there must be an an infinite number of them.
THE DEMIURGIC ANTHROPIC PRINCIPLE
Perhaps the strangest version if the anthropic principle (hereby christened the Demiurgic Anthropic Principle, or DAP) holds that the reason our universe appears "fine-tuned" for life is because it was designed that way - not by God, but by alien scientists in another universe.
The reasoning behind this startling hypothesis is as follows: if it is true - as physicists like Andre Linde, Alan Guth and others claim - that a universe can be created in a laboratory, then the possibility arises that our own universe was created in a laboratory:
"...the only thing you needed to get a universe like ours started is a hundred-thousandth of a gram of matter. That's enough to create a small chunk of vacuum that blows up into the billions and billions of galaxies we see around us. It looks like cheating, but that's how the inflation theory works - all the matter in the universe gets created from the negative energy of the gravitational field. So, what's to stop us from creating a universe in a lab? We would be like gods!"
If "intelligent life takes over the business of making universes," as British physicist Edward Harrison predicts, then living universes could multiply rapidly:
"There are about 10 billion galaxies like our own Milky Way in the observable Universe. If, during the lifetime of each galaxy, a single civilisation emerges which makes a new universe - a modest figure when you consider that our Galaxy alone has 200 billion suns - then our Universe manages to reproduce 10 billion times! Furthermore, if intelligent life in each galaxy of each daughter universe repeats the ultimate experiment just once, the result is 10 billion times 10 billion granddaughter universes. And so on, ad infinitum."
If our universe was created by alien scientists whose own universe was also the result of an experiment in yet another universe, then where did the very first living universe of all come from? Per the Strong Anthropic Principle, there are only two possibilites: either a god created the first "living" universe, or else it arose by chance from an ensemble of "mostly-dead" universes - a multiverse.
So where did this god, or this ensemble come from? Harrison doesn't really try to answer:
"Perhaps the supreme being occupied another universe created by an even higher form of intelligence, and perhaps the initial ensemble consisted of botched and bungled creations by a sorcerer's apprentice in another universe."