I'm not sure I agree with the statement that what constitutes a legal person is necessarily subjective. In particular the law of estates is very much concerned with when a legal person ceases to be. Here is the definition of death in statute from IL, and I suspect many states have similar language:
In the dictionary death is defined as the end of life, so life must be present for there to be death. If the above is the legal definition of the end of life, then logically the existence of the two conditions in the statute above are the necessary conditions for a person to have life. Thus it follows that a human organism that exhibits both blood flow (circulation) with the exchange of gases in the blood (respiration) and brain functions at the most basic level is legally alive.
Engineers use extremes to prove their concepts and I do the same with mine.
You have given the potential human quite a ways to grow before calling it life.
Let's play with a scenario.
Let' say you and I are from an advanced civilization and come to earth to save the people from a plague that has infected all that have been born.
We have the technology to maintain life from conception on or from what I say begins live. When sperm and egg fertilize and begin to split. What I call a potential human.
Let's also say that we will be well paid for every human we can extract from people and grow to term.
I cannot see us only harvesting only those who "exhibits both blood flow (circulation) with the exchange of gases in the blood (respiration) and brain functions at the most basic level is legally alive" as legality does not apply to our harvest.
Would you say goodbye to all that cash you can get if you bring the less developed zygotes and fertilized and growing eggs. I would not.
Regards
DL
My example was in response to the statement that a legal person is subjective. I was not saying when there was biological life, but rather how one might define a legal person entitled to legal rights. The difference can be important, so let me provide you with an additional scenario.
An egg is fertilized and forms a zygote of undifferentiated cells. Suppose one were to call this a legal person as defined by the act of fertilization. It is not uncommon for live cells to flake off the zygote, just as they do from our own bodies. Sometimes those flaked-off cells spontaneously begin to form a new zygote, and thus an identical twin. In this case there was no fertilization to mark the beginning of the twin. So if fertilization defines the person, then the identical twin is not a person and has no legal rights, though I certainly think the identical twin should have rights.
Since that's not a satisfying conclusion, let's travel an alternate path. Suppose that the twin is a person at the point the cell is shed from the zygote. We don't know a priori which shed cells will develop into new zygotes, but it is presumably rare based on the number of identical twins. Therefore to protect the identical twin as a legal person at the moment the cell is shed from the original zygote, we would have to ascribe personhood to all shed cells from zygotes, most of which never develop. That creates the prospect of not one or two but many persons in the form of separate zygotic cells in the womb at the same time. As they die off, which most will, they now potentially have legal issues that must be resolved as would any person who dies. This also seems an entirely unworkable definition to me.
One path of definition fails to recognize humans that should have rights, and the other path recognizes cells that would never reach a point that should have rights. Instead I offered one possible consistent test that can be applied to determine when a legal person exists.
Most cells that are shed are dead but that aside, in my description of a potential human, I included dividing and joining cells so I covered my rump on that.
You introduce the issue of rights which is a completely different issue.
Rights are subjective and not objective so that is a whole new ball game.
The rights a zygote might be given depends entirely on what the community says they are as it is to the community to enforce or grant those rights.
In the days of city states and finite resources, babies were often sacrificed because to let them live would have brought hardship to the workers needed to bring in those finite resources.
If too many babies lived, adults would die. Their morality, I guess, said that the good of the whole tribe was more important than a new baby.
To give the ancients points for trying to reduce the numbers of those sacrificed babies, let us not forget the Temple Prostitutes and the elaborate mating rituals of husband and wives that some tribes resorted to to keep those sacrifices at a minimum.
Regards
DL