Leaving aside therefore all the scientific books which teach us only to see men as they have made themselves, and meditating on the first and most simple operations of the human soul, I believe I perceive in it two principles that are prior to reason, of which one makes us ardently interested in our own well-being and our self-preservation, and the other inspires in us a natural repugnance to seeing any sentient being, especially our fellow man, perish or suffer. It is from the conjunction and combination that our mind is in a position to make regarding these two principles, without the need for introducing that of sociability, that all the rules of natural right appear to me to flow; rules which reason is later forced to reestablish on other foundations, when, by its successive developments, it has succeeded in smothering nature.
In this way one is not obliged to make a man a philosopher before making him a man. His duties toward others are not uniquely dictated to him by the belated lessons of wisdom; and as long as he does not resist the inner impulse of compassion, he will never harm another man or even another sentient being, except in the legitimate instance where, if his preservation were involved, he is obliged to give preference to himself. By this means, an end can also be made to the ancient disputes regarding the participation of animals in the natural law. For it is clear that, lacking intelligence and liberty, they cannot recognize this law; but since they share to some extent in our nature by virtue of the sentient quality with which they are endowed, one will judge that they should also participate in natural right, and that man is subject to some sort of duties toward them. It seems, in effect, that if I am obliged not to do any harm to my fellow man, it is less because he is a rational being than because he is a sentient being: a quality that, since it is common to both animals and men, should at least give the former the right not to be needlessly mistreated by the latter.I'd forgotten about this until preparing again to teach Rousseau in my Honors Humanities courses. Since I was at the same time discussing Kant and Bentham in my animal ethics class, I had them look over the paragraphs above as well. The end of the passage anticipates Bentham's remarks, and also helps, I think, in raising questions about Kant (and his grounding moral considerability in rationality).
Of course, there's much else that could be discussed. Rousseau is positing those two principles as our basic moral psychology--and as I understand him, the moral psychology of "original man," prior to life in society, and language (and reason, in any sense above the kind of reasoning other animals are capable of). And so then one might wonder whether Rousseau is attempting to derive an ought (in the second paragraph) from the is of our, as it were, prehistoric psychology. Still, that seems like a fine place to start: what are we "really" like; what has society foisted upon us? Of course, since his "history" of our development is "conjectural," there are problems here. As he allows, perhaps we never existed as solitary beings. (Does that obviously undermine either of the two principles above, self-preservation and pity?)
Another thing that strikes me is what he says about "the participation of animals in the natural law." He says that animals cannot follow the law because they lack freedom, but that we still should not mistreat them. But when he is describing the relationship between humans in (his version of) the state of nature and other animals, he stresses that predators tend not to attack other predators, and that the threat posed to them by robust, self-sufficient human beings would have led such animals not in general to tangle with humans. (And predators do not prey upon more than they need.) So it would seem that such animals participate (if not self-consciously) in at least the first law (of self-preservation). But if there's something that such animals lack that prevents their--natural, instinctual--participation in the second law (pity)--understood as a psychological law--it wouldn't seem to be intelligence or freedom that's relevant, but rather pity. Of course, there are now various studies that suggest that some animals exhibit "empathy" toward other members of their species when those animals are hurt, and so those animals would seem to exhibit both of the traits that Rousseau identifies as fundamental in our own (original, natural) moral psychology. We could then, I suppose, say that such animals are "moral animals"--which is just to say that their moral psychology (or, behavior) is somewhat like our own. And more like us--or some of us--to the extent that the capacity for empathy crosses species lines. We might say that their "participation" in the natural law is merely involuntary (instinctual). But on Rousseau's picture, that would have been true of the original humans, too.
In some sense (and this is perhaps too quick), Rousseau's ethic amounts to something like this: act like an animal (and bear in mind that animals do not have the various false needs that have been engendered by egocentrism and society). Or: act like the kind of animal that you (really) are. Our freedom, it turns out, often gets in the way of that, which is peculiar, since it implies that what we are is the animal that has a hard time acting like itself (or its "true" self).