Dignity, humanity and Aristotelianism

1. All humans have dignity because they are humans.
2. Humans do not have dignity because of an extrinsic property.
3. So, being human isn't an extrinsic property.
4. If to be human is to be a member of a particular biological taxon, then being human is extrinsic. (Biological taxa are defined by gene interchange in a population and are thus extrinsic characterizations of individuals.)
5. So, to be human is not the same as to be a member of a biological taxon.
6. Our best alternative to the biological taxonomic account of what it is to be human is the Aristotelian account that it is to have a human form, so the Aristotelian account is probably true.