First of all, I’ll describe the anthropic principle.  It is a theory that one of the ways you can explain something is by the sole fact that you are observing it.   It is a form of cogito erg sum, except instead of the small deduction by Descarte, someone using the anthropic principle is able to use it to explain the universe.

Let me give an example.  In our solar system, the earth is third planet from the sun, in the so called habitable zone, where liquid oceans cover the surface of the earth.  The fifth planet is Jupiter our largest planet a gas giant that most astronomers believe is a sort of guardian, deflecting many rogue asteroids and comets away from earth.   Life began almost as soon as the planet cooled down from its molten state.
Now to someone using the anthropic principle there are several questions you can pose, such as, why do we live on the third planet from the sun, and not mars or venus.  The answer is that if the earth were not in the exact right position in the solar system, life would not have evolved, and we would not be around to observe it.   Why does Jupiter orbit as the fifth planet?  According to the anthropic principle, since we evolved on the third planet, without Jupiter guarding us against cosmic bombardment intelligent life would never had a chance to evolve.

What about the constants and laws of nature, gravity, dark energy, dark matter, the form and shape of the universe, the fact that the fine structure constant is almost 1/137?  All of these can be explained by use of the anthropic principle.  Coincidences become explained.   And from there, you can start believing that if everything that has happened to create life is so incredibly lucky, then there must have been a multiplicity of universes, since otherwise this incredibly improbable event of life is just too coincidental either that or you have to believe in gods or space aliens creating our universe.  To quote Candide we live in the luckiest of universes.

That’s where the anthropic principle leads you.  But it is all circular.  In order to draw any conclusion about life, the universe, Jupiter, the earth, you need more than one data point.  The single data point “I exist” leads only to one conclusion, “Therefore I exist.”  There is nothing else you can draw from it.  If, in using the anthropic principle you are led to the exact opposite conclusion a year later when new evidence comes in, does this suddenly mean the anthropic principle is wrong and that your previous conclusion was proof of inconsistency?   Why no sir, suddenly, everything is explained but now the anthropic principle explains the opposite conclusion.

Let me give you some examples.  There has not been a lot of work studying models of how much Jupiter does or does not protect the innner planets, but Jonathan Horner of Great Britain’s Open University has been building models and his conclusion has been the opposite of the currently fasionable position of Jupiter being the guardian planet.  Instead his models seem to show that Jupiter’s perterbation of the objects surrounding the solar system increases the collisions with earth instead of decreasing the number of collisions.  Obviously more work needs to be done, and this is a case where sufficient computer modeling will give us the answer.  For now though let’s assume Horner is right.  What does this mean for the anthropic principle?
Remember that the anthropic principle used to state that we needed Jupiter to be the 5th planet from the sun in order to deflect away asteroids and comets.  But now the opposite is true.  We need Jupiter as the 5th planet not to deflect comets away from earth but to deflect comets towards earth, supplying earth’s great oceans and giving the earth bursts of destructive evolutionary advances since without the chicxulub asteroid dinosaurs would still roam the earth.
The anthropic principle explains everything.  It is like Marxism that way as Karl Popper has said, by re-interpreting the theory in order to make it agree with whatever evidence there is, the theory becomes both useless and irrefutable.
Ok, how about the earth having to be the 3rd planet from the sun in order to have liquid oceans and thus life?  Well as we learn more and more about the planets, we are finding most of the liquid water of planets is not on the surface but under a frozen thick layer of ice.   And that for much of the history of the planet earth itself, we were a snowball planet.  Thick with layers of ice, but under the ice at the bottom was a layer of water.
The moon Europa is exactly like this right now.  The entire moon  is covered with a layer of ice 3 or 4 miles thick, and under this layer of ice is a lukewarm ocean.
Some think that it was the most recent snowball earth episode which triggered the advent of multicellar life.  Twenty years ago most biology textbooks would have given you warm “ponds” of ancient earth as the origin of life.  But more recent textbooks will give you the origin of life at the bottom of the oceans in the “smoker” chimneys where strange animals and planets life, powered not by the sun but by the chemical power of geothermal vents.  Thanks to DNA sequencing we now know that Archaea is the most primitive life on earth, and the place to find these organisms are in the extremeophile places, including deep under the earth in the planets warm moist rocks.
Life never ended when eath became a snowball, it continued to evolve under the icy crust.  So what does the anthropic principle say now about the need for liquid oceans?   If the liquid oceans are not needed, what do you do with your previous theory that earth had to be the third planet.

What if we discover life on Europa?  And what if we discover that life is common through the universe and not just on “habitable zone planets” but on every planet big enough to hold geothermal heat and that it is geothermal activity that creates both water and life and not the sun?
Well, then you can just change your theory to fit the data.  Now instead of saying that liquid water on the surface is required for life to exist at all, you can say that it is liquid water that is required for life to look up at the heavans.  So maybe life does exist on Europa, and a million other planets in the universe, but maybe those planets are all snowballs and their life didn’t get a chance to peer up at the heavens.  They are stuck swimming under the ice with flippers instead of hands, and now you can use the anthropic principle to explain why we don’t have space aliens visiting us all the time, it’s because they’re all fish on alien snowball earths.
It’s a seductive process, because the anthropic principle seems to explain everything, but in explaining everything it explains nothing.

We shouldn’t try to philosophize our way to science, which is all the anthropic principle is.  Instead we need to build probes, hitch up our pants, get on a rocket go out there,  and step onto the strange alien worlds.  That’s how we are going to discover why the earth is like it is, what the universe is made of, and what it all means.

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