It also suggests that there are only two cases, animals with full human
rights and animals with none. Actually it would make more sense to
propose that animals should have some rights. Most states protect
animals against cruelty, for example, while still allowing them to be
humanely killed.
Some humans, such as children and the mentally disabled, don't respect
property rights and are not able to commit to contracts, but we do accord
them limited rights. So this ability in itself does not seem to be the
basis on which we draw the line.
Hal