A different way of summarizing telos that may be helpful when discussing this topic is that an object or agent fulfilling it's telos is supposed to be the most fully itself. So, for Aquinas and Aristotle, because the quality that most clearly sets off the category 'human' is "a reasoning animal" actions that interfere with that part of human identity are unnatural and interfere with telos.
The reason for this is because of the etymology of the word "telos". It has a "common ancestor" so to say with the word teleos which means perfection. The background for this is that we are dealing with words that had a particular meaning in pre-Greek language/culture. A culture that for some reason linked those words. Fast forwarding 3,000+ years that inference doesn't make sense to modern readers.
Today's post, Three Fallacies of Teleology was originally published on 25 August 2008. A summary (taken from the LW wiki):
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