In a 2002 article on stem cell research in The Public Interest , Leon Kass offered a gruesomely memorable test for the claim that a human embryo is nothing but a piece of tissue. On the one hand, he noted, if an embryo dies “we are sad—largely for her loss and disappointment, but perhaps also at the premature death of a life that might have been. But we do not mourn the departed fetus, nor do we seek ritually to dispose of the remains.” Thus, from this angle, “we do not treat even the fetus as fully one of us.”

Kass offers another test: “we would, I suppose, recoil even from the thought, let alone the practice—I apologize for forcing it upon the reader—of eating such embryos, should someone discover that they would provide a great delicacy, a ‘human caviar.’ The human blastocyst would be protected by our taboo against cannibalism, which insists on the humanness of human flesh and does not permit us to treat even the flesh of the dead as if it were mere meat. The human embryo is not mere meat; it is not just stuff; it is not a ‘thing.’ Because of its origin and because of its capacity, it commands a higher respect.”

How much higher, Kass asks? He answers that his inclination is to say that the embryo does not deserve the same respect as “a fully developed human being,” but admits he may be wrong: “Indeed, there might be prudential and reasonable grounds for an affirmative answer, partly because the presumption of ignorance ought to err in the direction of never underestimating the basis for respect of human life (not least, for our own self-respect), partly because so many people feel very strongly that even the blastocyst is protectably human.”

We don’t, in short, really believe that the embryo is “only tissue.” The danger with the human caviar argument is that some may be prepared to be manically consistent.