The form of design arguments from nature generally follows the steps below. If this pattern is not explicit, its elements are generally understood from the flow of the discussion.
1. The positive case for design: certain features of the natural world appear designed. This is based on what we know about the feature, and what we know generally about things that are designed and things that are not designed. Based on this knowledge, we make an inference of design.
2. The counter-argument: Known natural processes could produce the feature (without design). To make the argument, you need to explain why the natural processes could produce the outcome, and why they are the most plausible explanation.
3. The reply to the counter-argument. The known natural processes are not sufficient to produce the feature, or it is not plausible that the natural processes produced the feature. If this argument succeeds, you return to the original inference based on the positive evidence from #1.
**************************
NOTE: This a tentative draft. I may make changes based on suggestions and comments from others.
**************************
NOTE 2:
“(without design)” in “2. The counter-argument: Known natural processes could produce the feature (without design).” was added on Jan 28, 2010.
Well stated, succinct and clear.
[...] I already did in other comments. Also here. [...]
[...] More here. [...]