Hey Terry,
>>There is only one Earth with exactly what Earth has or had at a particular time in a particular way.
What's your basis for that statement? And how are you defining "unique"? For instance, I could draw a scribble on a piece of paper and it could be totally unique in the universe. But it could also be considered just another scribble.
I'd grant that earth is unique in the same sense as my scribble. But is it unique in ways that matter for forming life? If you're arguing that we can't know whether it is or it isn't, then I agree with you.
>>The fact that there are billions of similar (not identical) planets is not enough.
If you found a coin that you'd never seen before, would you assume it was the only one on the planet? If you decided to test your assumption, would you consider it sufficient to scour a 10-foot square patch of land? The fact that we haven't found life anywhere else we've looked is meaningless, since we've only looked in a few places, and our ability to look there is still pretty weak.
>>If the Earth were closer or farther from our Sun there could be no life. Our placement is unique.
I seriously doubt this one. Life exists on our planet in all sorts of temperatures and environments. I wouldn't venture a guess on how far it could be shifted and life still be viable, but there is certainly a range.
>>The difference between the POTENTIAL for life elsewhere has nothing to do with the ACTUAL probability of life happening.
I'm not sure I can agree with this. If the potential is there, doesn't it automatically raise the probability? Having potential raises it from impossible, to possible. However the underlying question is "Does the potential exist?" and without knowledge of how life arose spontaneously (if it did at all), we can't say with certainty that the potential exists.
Dave