Unlike [John] Rawls, whose considered views on our duties regarding animals are unclear at best, [Immanuel] Kant provides us with an explicit statement of an indirect duty view. That Kant should hold such a view should not be surprising; it is a direct consequence of his moral theory, the main outlines of which may be briefly, albeit crudely, summarized. . . . On Kant's view, rational beings, by which he means moral agents, are ends in themselves (have, that is, independent value, or worth, in their own right, quite apart from how useful they happen to be to others). As such, no moral agent is ever to be treated merely as a means. This is not to say that we may never make use of the skills or services of moral agents in their capacities as, say, mechanics, plumbers, or surgeons. It is to say that we must never impose our will, by force, coercion, or deceit, on any moral agent to do what we want them [sic] to do just because we stand to benefit as a result. To treat moral agents in this way is to treat them as if they had no value in their own right or, alternatively, as if they were things. As Kant remarks, "beings whose existence depends, not on our will, but on nature, have nonetheless, if they are non-rational only a relative value and are consequently called things." Moral agents are not nonrational, do not have "only a relative value," and are not things. Moral agents (rational beings) are ends in themselves.
(Tom Regan, The Case for Animal Rights, updated with a new preface [Berkeley and Los Angeles: University of California Press, 2004], 174-5 [italics in original; ellipsis added; endnote omitted] [first edition published in 1983])