Zygotes, souls, substances, and persons.

The thesis that the human zygote is essentially identical with the person into which it can develop is difficult to maintain, because the zygote can become several persons. In addition, the thesis depends upon ambiguities in the notions of human being, human individual, human body, and soul. A human being may be individual in the sense of either a biologically integrated unity or a psychologically integrated unity. A person is a psychologically integrated unity, because it must unify its experiences in morally imputable actions. To say that the zygote is a person requires one to assert that the zygote has the same principle of psychological integration, i.e., a rational soul, as one who can obviously manifest psychological integration. The assertion is incapable of being justified in empirical (e.g., non-religious) terms.