(1) A right of A against B implies a duty of B to A.What is asserted in (1) is that A's having a right to have a certain individual act done to him by B implies a duty for B to do that act to A; (2) asserts the converse implication; what is meant by (3) is that A's having a right to have a certain act done to him by B implies a duty for A to do another act to B, which act may be either a similar act (as where the right of having the truth told to one implies the duty of telling the truth) or a different sort of act (as where the right to obedience implies the duty of governing well); (4) asserts the converse implication.
(2) A duty of B to A implies a right of A against B.
(3) A right of A against B implies a duty of A to B.
(4) A duty of A to B implies a right of A against B.
Of these four propositions the first appears to be unquestionably true; a right in one being against another is a right to treat or be treated by that other in a certain way, and this plainly implies a duty for the other to behave in a certain way. But there is a certain consideration which throws doubt on the other three propositions. This arises from the fact that we have duties to animals and to infants. The latter case is complicated by the fact that infants, while they are not (so we commonly believe) actual moral agents, are potential moral agents, so that the duty of parents, for instance, to support them may be said to be counterbalanced by a duty which is not incumbent on the infants at the time but will be incumbent on them later, to obey and care for their parents. We had better therefore take the less complicated case of animals, which we commonly suppose not to be even potential moral agents.
It may of course be denied that we have duties to animals. The view held by some writers is that we have duties concerning animals but not to them, the theory being that we have a duty to behave humanely to our fellow men, and that we should behave humanely to animals simply for fear of creating a disposition in ourselves which will make us tend to be cruel to our fellow men. Professor D. G. Ritchie, for instance, implies that we have not a duty to animals except in a sense like that in which the owner of an historic house may be said to have a duty to the house. Now the latter sense is, I suppose, purely metaphorical. We may in a fanciful mood think of a noble house as if it were a conscious being having feelings which we are bound to respect. But we do not really think that it has them. I suppose that the duty of the owner of an historic house is essentially a duty to his contemporaries and to posterity; and he may also think it is a duty to his ancestors. On the other hand, if we think we ought to behave in a certain way to animals, it is out of consideration primarily for their feelings that we think we ought to behave so; we do not think of them merely as a practising-ground for virtue. It is because we think their pain a bad thing that we think we should not gratuitously cause it. And I suppose that to say we have a duty to so-and-so is the same thing as to say that we have a duty, grounded on facts relating to them, to behave in a certain way towards them.
Now if we have a duty to animals, and they have not a duty to us (which seems clear, since they are not moral agents), the first and last of our four propositions cannot both be true, since (4) implies that a duty of men to animals involves a right of men against animals, and (1) implies that this involves a duty of animals to men, and therefore (4) and (1) together imply that a duty of men to animals involves a duty of animals to men. And since the first proposition is clearly true, the fourth must be false; it cannot be true that a duty of A to B necessarily involves a right of A against B. Similarly, the second and third propositions cannot both be true; for (2) and (3) taken together imply that a duty of men to animals involves a duty of animals to men. But here it is not so clear which of the two propositions is true; for it is not clear whether we should say that though we have a duty to animals they have no right against us, or that though they have a right against us they have no duty to us. If we take the first view, we are implying that in order to have rights, just as much as in order to have duties, it is necessary to be a moral agent. If we take the second view, we are implying that while only moral agents have duties, the possession of a nature capable of feeling pleasure and pain is all that is needed in order to have rights. It is not at all clear which is the true view. On the whole, since we mean by a right something that can be justly claimed, we should probably say that animals have not rights, not because the claim to humane treatment would not be just if it were made, but because they cannot make it. But the doubt which we here find about the application of the term 'rights' is characteristic of the term. There are other ways too in which its application is doubtful. Even if we hold that it is our duty not merely to do what is just to others but to promote their welfare beyond what justice requires, it is not at all clear that we should say they have a right to beneficent treatment over and above what is just. We have a tendency to think that not every duty incumbent on one person involves a right in another.
(W. D. Ross, The Right and the Good [1930; repr., Indianapolis/Cambridge: Hackett Publishing Company, 1988], 48-50 [italics in original; footnote omitted])