This circle can be very small, including just your kin and those with whom you interact on a daily basis, or it can be extremely broad, including all humans, but also fetuses, animals, plants, and even the earth itself. For most of us, the circle is mid-sized, and working out its precise boundaries – does it include stem cells, for instance? – can be a source of anguish and conflict.
这种两元论极大地影响了我们的思想、行为和感觉。哲学家彼德·辛格论述了一种道德循环的概念—一些对我们具有重要道德意义的生物的循环。这个循环可以很小，只包括我们的亲朋好友和那些与我们每天打交道的人；或者它也可以很大，不仅包括全人类，还包括胚胎、动物、植物甚至地球本身。对我们大多数人而言，这个循环是中等大小，界限分明—比如，它是否包括干细胞？--能够成为痛苦和冲突的源头。