Stages of life : a new metaphysics of conceptionism

When a human being comes into existence is crucial in bioethics. Conceptionism is the view that a human being comes into existence at conception. The twinning argument is an influential objection to this view. All versions of the twinning argument rely on a metaphysics of material objects, namely, endurantism. Given this, a strategy for defending conceptionism against the twinning argument is to deny endurantism and adopt an alternative metaphysics of material objects. A version of this strategy which has been debated in this journal is to adopt perdurantism, or the ‘multiple occupancy view’, on which monozygotic twins share the zygote region as a temporal part. We present a novel version of this strategy: conceptionists can evade the twinning argument by adopting an exdurantist metaphysics of material objects. We suggest reasons for thinking that this is a plausible and, indeed, preferable way for conceptionists to avoid the twinning argument.