According to Catholic doctrine, the Father, the Son and Holy Sprit are three distinct persons even though they are one essence. Only one of those persons — Jesus Christ — is also a human being whose life had a beginning and an end.
I am not an expert in Trinitarian theology. But I mention it here because, great mysteries aside, this Catholic doctrine uses the notion of person in what, from our point of view today, is the standard way.
John Locke called person a forensic concept. What he had in mind is that a person is one to whom credit and blame may be attached, one who is deemed responsible. The concept of a person is the concept of an agent.
Crucially, Locke argued, persons are not the same as human beings. Dr. Jekyl and Mr. Hyde may be one and the same human being, that is, one and the same continuously existing organic life; they share a birth event; but they are two distinct persons. And this is why we don't blame the one for the other's crimes. Multiple personality disorder might be a real world example of this.
I don't know whether Locke believed that two distinct persons could actually inhabit the same living human body, but he certainly thought there was nothing contradictory in the possibility. Nor did he think there was anything incoherent in the thought that one person could find existence in multiple distinct animal lives, even if, as a matter of fact, this may not be possible. If you believe in reincarnation, then you think this is a genuine possibility. For Locke, this was no more incoherent than the idea of two actors playing the same role in a play.
Indeed, the word "person" derives from a Latin (and originally a Greek) word meaning "character in a drama" or "mask" (because actors wore masks). This usage survives today in the phrase "dramatis personae." To be a person, from this standpoint, is to play a role. The person is the role played, however, not the player.
From this standpoint, the idea of non-human, non-living person certainly makes sense, even if we find it disturbing. Corporations are persons under current law, and this makes sense. They are actors, after all, and we credit and blame them for the things they do. They play an important role in our society.
Is a fertilized human egg alive? I would say yes. Is it a human being? This is much harder to answer and is comparable to the question, is a sapling already an oak tree? However we answer it, though, it is distinct from the question whether we should extend legal protections to the fertilized eggs of human beings? And none of these questions should be confused with the question whether the fertilized egg is a person, which it is clearly not.
It is a good thing that the recent effort in Mississippi to extend "personhood" to fertilized human eggs was unsuccessful at the polls. Popular will cannot succeed in making a fertilized egg a person.
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