I’m going to go out on a limb and suggest that many C. S. Lewis fans–maybe especially his many evangelical admirers–don’t know that Lewis wrote a pamphlet for the British Anti-Vivisection Society. This essay, reprinted later in God in the Dock, anticipates some key arguments since made by philosophical proponents of animal rights.
Lewis posits a dilemma for the defender of animal experimentation: either they hold that humans are metaphysically superior to (non-human) animals (he identifies this with the Christian view), or they believe that there is no inherent metaphysical difference between humans and other animals (he calls this the naturalistic or Darwinian view).
If one takes the first view, Lewis argues, it by no means follows that humans are entitled to treat animals any way they wish. “We may find it difficult to formulate a human right of tormenting beasts in terms which would not equally imply an angelic right of tormenting men” (“Vivisection,” God in the Dock, reprinted in The Collected Works of C. S. Lewis, p. 452). Or, we might add, an extraterrestrial right of tormenting men. Further, “we may feel that though objective superiority is rightly claimed for man, yet that very superiority ought partly to consist in not behaving like a vivisector” (p. 452). This turns the superiority argument on its head; Andrew Linzey has also made much of this line of thinking (see his Animal Theology and Why Animal Suffering Matters, among other works).
Of course, as Lewis notes, most of those who experiment on animals are not Christians with a belief in the metaphysical superiority of humanity, but Darwinian naturalists who see humans as just one species of animal among many (albeit one with certain unique characteristics). “We sacrifice other species to our own not because our own has any objective metaphysical privilege over others, but simply because it is ours” (pp. 452-3). But Lewis is quick to point out that if there is no great metaphysical gulf separating human from non-human animals, what reason is there to draw the line at the species barrier? If all that justifies our preference for our own species is sentiment, than wouldn’t sentiment also justify a preference for our own nation, class, or race? “Once the old Christan idea of a total difference in kind between man and beast has been abandoned, then no argument for experiments on animals can be found which is not also an argument for experiments on inferior men” (p. 453).
This argument similar to the one known as the “argument from marginal cases”–i.e., many non-human animals have the same cognitive abilities as so-called marginal humans (e.g., the severely mentally handicapped), so why are we justified in, say, experimenting on animals but not in experimenting on “marginal case” humans?
Of course, it’s open to the naturalist to admit the force of the argument and accept that we aren’t justified in experimenting on “marginal” humans or non-human animals. (James Rachels takes such an approach in his book Created from Animals: The Moral Implications of Darwinism.) But this just concedes Lewis’s main point: once you do away with the “metaphysical” distinction between humans and animals, there is no rational ground for treating humans and non-human animals differently simply because they belong to different species.
A Thinking Reed 
Huh, I didn’t know that about Lewis. Time to go find that essay, and pass it out to my pro-Lewis friends and family.
But this just concedes Lewis’s main point: once you do away with the “metaphysical” distinction between humans and animals, there is no rational ground for treating humans and non-human animals differently simply because they belong to different species.
The difference though between seeing this from the “marginal cases” perspective and Rachels’ is, however, that the latter recognizes what much of science is discovering and what some of us who live with fellow creatures know, that the capacities we associate only with human beings are rather on a scale and close to us on that scale are some fellow creatures, and hence, given the capacities of both ourselves and our fellow creatures for pain, pleasure, fear, and yes, love, it is wise to err on the side of drawing a “fence around the law” (to use a Jewish expression) and recognize our relatedness in flesh as creatures by not allowing for experimentation. One wants to make us primarily into crown of creation, the other wants to make us primarily into related responsibles. The distinction is crucial in my opinion.
The latter, I would add, comes closer to a theologia crucis or incarnationis as the case may be, by placing our earthiness, that is, humility, as the core of our relationships with fellow creatures, rather than our sense of superiority (glory). There is One crowned and he was born in a manger and died on a cross surrounded in my mind by a few humans and many other creatures of clay.
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