Does Observer Selection Explain Earth's Habitability?
We necessarily observe a planet compatible with our existence. If Earth had never become habitable, we would not be here to notice its absence of habitability.
This creates an observer-selection effect. Any beings capable of investigating their planet will inevitably find themselves in an environment that permitted observers to arise.
Earth's suitability for life may therefore appear remarkable partly because we cannot observe ourselves from one of the countless worlds on which observers never developed.
Why This Matters
Observer selection warns us against arguing that Earth must have been purposefully arranged simply because we observe ourselves living on a habitable planet.
Our observation is not a random sample from all planets. It is necessarily drawn from the subset of environments capable of producing observers.
Any argument from Earth's habitability must therefore take this selection effect into account.
Response
Observer selection explains something important: we should not be surprised that observers find themselves somewhere compatible with observers.
It does not by itself explain why environments capable of producing observers exist, how frequently they occur, or why the underlying laws and planetary processes permit such environments.
The distinction is between explaining our observation of habitability and explaining the origin of the conditions being observed.
Observer selection therefore qualifies a design argument, but it does not make the underlying scientific questions disappear.
Question That Remains
After accounting for observer selection, what—if anything—can the existence and character of habitable worlds tell us about why life-permitting environments exist?