Joined: June 2004
If you were simply referring to "similar causes produce similar effects" then the choice of an artist is an odd one. Naturally the "similar" elements produced by an artist would tend to be stylistic and thus we would expect aesthetic commonalities rather than functional ones.
Artists of all people would be the ones who most greatly undermine the simplistic use of the metaphysical principle you refer to, to apply to the output of intelligent designers.
But what amazes me most is the fact that you consider the fact that Common Descent provides strong grounds for these partiocular elements to be conserved to be a seperate issue from the fact that the Common Design does not. But this comparison is a clear indication that Common Descent is the better explanation for the conservation of these features. It is only by comparisons like this that we can determine which explanation is the better.
But most amazing of all is the final sentence which indicates that Common Design offers a tautology in place of explanation.
There are valid responses you could have offered - for instance an explanation of why Common Design WOULD predict that these particular elements would be conserved. You could have looked for other common elements that better fitted the Common Design argument. BUt you cannot claim that a prediction that maybe some elements will be common to all life is as good as a prediction that specific elements are very likely to be common to all life.