The second definition is no more arbitrary than the first. The first depends on what one would accept as "rational," the second merely depends on what one would accept as "human." The second is far easier to define.
i'm no respecter of Bentham, and in fact am contemptuous of him.
Easier to define perhaps, but irrelevant in itself. Its only relevance is that being human is a workable proxy to being rational, and that being rational is relevant to having rights. Choosing the second approach, then, does not relieve you from working out what it means to be rational, and why being rational is relevant to having rights. To pretend otherwise is definition-laundering --- too clever by half, and not worthy of your intellect.
As for the charge of "definition-laundering," I have no idea what that means.
His was goofy--he decided what human nature was, on no particularly justifiable basis, and then defined utility on that basis. His "panopticon" was institutionalized torture on a large scale (the first panoticon prison was in Pennsylvania--Philadelphia, i believe). His contention was that men kept in total isolation in his system would have no choice but to reflect on their wickedness, and would thereby be brought to repentance.
The Panopticon is widely, but erroneously, believed to have influenced the design of Pentonville Prison in North London, Armagh Gaol in Northern Ireland, and Eastern State Penitentiary in Philadelphia. These, however, were Victorian examples of the Separate system, which was more about prisoner isolation than prisoner surveillance; in fact, the separate system makes surveillance quite difficult.
"I define the class of 'evildoers' to comprise all serial murderers in America, plus joefromchicago. Evildoers, as a class, deserve capital punishment. Joefromchicago, being a member of the class, therefore deserves capital punishment."
This manifests a form of intellectual cheating that I call definition laundering.
Thomas wrote:"I define the class of 'evildoers' to comprise all serial murderers in America, plus joefromchicago. Evildoers, as a class, deserve capital punishment. Joefromchicago, being a member of the class, therefore deserves capital punishment."
That's a perfectly logical statement. The question remains whether the distinction is defensible, not whether we are entitled to make distinctions in the first place.
Frankly, I don't see why the proposition "this person isn't rational, so he has no rights" is any less arbitrary than the proposition "this rabbit isn't a member of a class that is capable of rationality, so it has no rights."
If you think the former is a better way to proceed than the latter on a utilitarian basis, however, I'd be interested in knowing why.
The second approach you suggested has the same problem. It pretends that being rational is necessary for having rights, but accords rights to humans without reason anyway. That, too, is logical malpractice, no matter if you believe that rationality is relevant to having rights.