Selection Effects
The Explanation
There is an unavoidable selection effect whenever we examine the habitability of our own planet.
We should not be surprised to observe that Earth permits life, because if Earth had never become habitable, we would not be here to observe it.
This reasoning cautions us against treating every favorable feature of Earth as independently surprising simply because it is favorable to us.
Supporting Evidence
Earth is presently the only inhabited world known to science. Every observation we make about an inhabited planet therefore comes from the same example.
Our existence necessarily selects a planet on which at least one successful history of habitability and biological development occurred.
This makes observer selection an important consideration whenever probabilities are assigned to Earth's properties.
Limitations
Observer selection explains why observers necessarily observe conditions compatible with their existence. By itself, however, it does not explain how those conditions originated or how frequently they occur.
If habitable planets are common, observer selection may require little additional explanation. If the necessary combination of conditions is exceedingly uncommon, the underlying reason for that rarity would remain an important question.
Questions That Remain
How common are planets capable of producing observers?
How broad is the range of environments in which intelligent observers could arise?
Can meaningful probabilities be assigned when our only known inhabited example is Earth?
Intelligent Design Response
Intelligent Design should take observer selection seriously. We should expect to find ourselves in an environment compatible with our existence.
But that observation does not by itself explain why the universe contains planets capable of supporting observers in the first place.
The design question therefore remains focused upon the origin and character of the life-permitting system rather than merely upon the fact that we observe it.