Does having a brain make one a person? Does sentience?

Someone said recently that a human fetus cannot be considered deserving of full moral respect until he or she develops a central nervous system and cerebral cortex. But imagine a rational, intelligent alien race whose physiology is radically different than ours. These aliens are clearly persons who ought to be treated as such, but they do not have brains or nervous systems.

The abortion defender clarified that it is not the nervous system or brain itself that matters morally, but what it allows someone to do: experience pain and pleasure (i.e., have a kind of sentience, the capacity for pain/pleasure). But imagine again an alien race, this time one that -- though in every other way like us -- cannot experience feelings. (Think, to use an imperfect example, of the Vulcans of Star Trek.) Or imagine a person whose brain has been surgically altered to prevent the experience of pain and pleasure. Or consider people with congenital insensitivity to pain, a real condition. If these people are nevertheless people, beings who ought not be killed without just cause, then sentience is not necessary for one to be a person with a right to life.

Moreover, many non-human animals (probably including some insects) have a capacity to suffer pain and enjoy pleasure, but that fact does not necessarily preclude killing them, for they do not have the moral status of persons. So sentience seems neither necessary nor sufficient for a being to have full moral worth.

In addition, some people have more sentience than others, and a person can become more or less sentient. As Christopher Kaczor observes, "The kung fu master can put his arms around a burning cauldron ... The proverbial princess cannot stand the pea under her multiple mattresses." If moral worth depends on sentience, then some people are more valuable than others, and a person can become more or less valuable and deserving of respect.

Consider a clear-cut example: A man gets in a car accident and suffers permanent brain damage, but not enough to prevent him from functioning as a typical member of society. Nevertheless, some of his mental faculties have been slightly diminished. If those mental faculties are what confer moral worth, then he has become less valuable and less deserving of protection from being killed.

If this conclusion is false -- as anyone committed to the basic equality of all persons must hold -- then it is not true that moral worth depends upon having sentience, much less a brain or central nervous system. (Go here to learn about the only rationally sustainable basis for moral worth.)