Exactly.....
With a negative proof. One can demonstrate that all other reasonably possible theories are false based upon scientific arguments.
Just as it is possible to point out the internal contradictions of all non-theistic philosophies (like materialism, Hegelianism, pantheism, relativism), it is logically possible that all non-creation theories of human origins can be demonstrated scientifically to be false.
This is the approach that the ID movement, and Johnson in particular, is using (and Medved here). Johnson is content to demonstrate the impossibility of evolutionary theory without providing a definitive "solution" for human origins. He's content to leave the subject of human origins as an open question, which it is, given our very limited knowledge.
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On the other hand, we can positively proove the existence of a Creator through unaided reason. But the proof is logical, not scientific.