For human eros is the fruit of the peculiar conjunction of and competition between two divergent aspirations within a single living body, the impulse to self-preservation and the urge to reproduce. The first is a self-regarding concern for our own personal permanence and satisfaction; the second is a self-denying aspiration for something that transcends our own finite existence, and for the sake of which we spend and even give our lives. Other animals, of course, live with these twin and opposing drives. But only the human animal is conscious of their existence and is driven to devise a life based in part by the tension between them, in part of the fact that he does not fully understand what it is that his embodied life “wants of him.” In consequence, only the human animal has explicit and conscious longings for something higher, something whole, something eternal, something that we would not have were we not the conjunction of this bodily “doubleness,” elevated and directed upwards through conscious self-awareness.
Leon R. Kass – Life, Liberty, and the Defense of Dignity: The Challenge for Bioethics p.19