What you really seem to be complaining about here is the lack of ability to first AGREE on a definition, then stick to it when the definition is used against you. Quite frankly, given how you're referencing socrates without getting into plato's metaphysics (which deals exactly with your argument, trying to define what a person is given that physical characteristics can change.)
If I define a person as a being capable of building tools and using them... but you insist that only HUMANS can be a person, well that creates a categorical difference between our definitions. From your definition an extraterrestrial that comes in using a space ship isn't a person... by my definition many species of birds are also persons.
The underlying complaint here is that you THINK you made a good argument because you hold a different set of definitions than another person does. In real logic, establishing common definitions and beliefs is key... there is no way I am to know what YOU believe, hell you could disagree with everything I think you would agree with. There IS NO WAY for me to say "yep... I made a good argument because the only way Eliezer Yudkowsky can refute it is by deflecting and ignoring what I said... cause since it's obviously correct to me it must be obviously correct to everyone right? (And there are a large amount of people who actually believe in self-evident truth and don't recognize it as a fallacy)
Let me emphasize that a self-sealing argument (which is similar to what you're trying to argue) is a fallacy. If you agree a definition is correct, someone uses that definition against you to question your belief ("featherless chicken is a human" why not) but the flow of logic is such that by the definition you agreed upon a featherless chicken does fit in with what a human is... then changing your definition ex post facto is using a logical fallacy.
A "by definition" argument is one of the strongest arguments you can make, in the socrates claim about a chicken is close enough (really sounds like we're about to get into plato's metaphysics) to be an obvious attempt to show a weakness in the definition from which if socrates honestly believed featherless chickens should be classified as humans then the accepted definition would have supported the claim. Instead socrates is clearly trying to point out that the definition is faulted and offers a point that both debaters would accept is false (a featherless chicken is not a human).