Very good question and a really invigorating discussion!! The issue of whether the stages of development from zygote through fetus qualify as "life" or "person" has come up a few times, so I want to contribute to that discussion. I will leave discussion of morality aside.
I have spent years researching and writing about this topic; as a physician, I needed to know with absolute certainty how to define "life" and "person" before I became involved in contraception/prenatal/delivery/postnatal healthcare.
This is my secular argument that the definitions of "life" and "person" apply to all stages of the human organism's development.
I will use the colloquial term "person." But you can insert "an individual living human" instead of the word "person" at any point, and it will retain full meaning.
There are many situations where the criteria for "life" vs. "non-life" or "person" vs. "non-person" absolutely *must *be defined - at the very least for legality and bioethics purposes. Lacking a clear definition of "life" or "person" results in inconsistencies in discussions on policy and legality.
There is a continuum of possible definitions, many of which would have very different results and implications if selected to define "person." These definitions are necessarily binary; "half-person" or "3/11ths of a person" cannot logically apply.
I need these definitions to be based solidly on logic and scientific evidence.
I refused to accept any definitions the depend on current laws, belief systems, or emotions - these shift and change. The definition of a living member of our species cannot be based upon what is convenient, nor contingent on what we want the answer to be in order to justify how we feel about it.
I am not satisfied with ambiguity when it comes to a definition of human life.
A human organism, like all organisms, passes through stages in its development.
Tracing it backwards from end to beginning:
The reason for establishing the zygote as the initial stage of human development is that it is the earliest state in which the organism has its own unique DNA and is capable of independently continuing through the rest of the human development stages (provided there is adequate nutrient media and environment). Regardless of the amount of support, neither an ovum nor a spermatozoon has that capacity individually.
If someone is to define one stage as "person" vs "not-person" (same as for "life" vs "non-life"), then they must define the criteria for the person/non-person distinction. If they are unable to pinpoint the moment in development where that distinction takes place, then they cannot logically define personhood or lack thereof. In that case, either all stages are people, or no stage is a person.
It must be a single moment, for the distinction is binary: person, not a person. Shades of gray like "half-person...3/11ths of a person...17/273rd of a person" make no sense, and therefore do not apply.
For example if we use birth, a process spanning space and time (often lasting hours and a displacement of 12-24 inches), as the criteria for a binary definition, we leave those hours and inches undefined. Any events occurring in that time will also be undefined. That lack of definition causes inconsistencies in discussions on policy and legality.
Therefore the task is to establish the precise moment of distinction between person/non-person - if there is one.
It is given that an adult, the final human developmental stage, is clearly defined as a person. Then tracing development backwards, what marker is used to determine this abrupt change from 0 to 1?
Physiologically, when studying the molecular processes throughout the stages of development of zygote through adulthood and senescence, it becomes apparent that all change is gradual.
During embryogenesis changes are most visually striking (as has been mentioned about the early stages looking like other vertebrate embryos). But the developmental processes that began then will end at different times in the life cycle. For example, the growth plates of long bones that begins during embryogenesis won't finally fuse until one's early 20's.
Biochemically, with trillions of chemical reactions going on at different rates in the metabolism of a body comprised of more than 10^27 atoms, with trillions of different DNA transcription and translation processes running at any one instant, is there anything that can be pinpointed as an abrupt change warranting the distinction between being a person and non-person?
The microscopic level is uncertain, so let's take a big step back and look at it macroscopically.
The common sentiment that birth is the distinguishing factor in defining person/non-person, as we started to discuss, must be examined in detail.
Birth is not a binary state with an on/off toggle switch.
Tracing backwards from infancy, using the scientific term "organism" to avoid confusion, consider each snapshot in the continuous process of moving from uterus to outside environment:
- Organism, fully external, cord severed, placenta delivered (remembering that placenta and cord are fetal tissue), no tissue remaining. ----Nearly everyone would agree that it clearly fits the definition of a neonate.
But if this is the very first moment at which the organism is defined as a "person," then EVERYTHING before that moment must be defined as a non-person.
- Organism, fully external, cord severed, placenta not delivered.
---Is it a neonate yet? Person or not?
- Organism, fully external, cord not severed. ---Neonate or fetus?
Is it a person yet?
- Organism, feet still in vaginal canal. ---Are
the feet fetus, while the rest of the body is neonate? Is it a
- Organism, feet still in cervix, legs in vaginal canal.
---Are the feet fetus? The legs fetus or half-fetus? The upper body neonate? What % of a person is this?
- Organism, head protruding from
dilated cervix. ---Is the head a neonate, the body a fetus? Head of
a person with the body of a non-person?
Organism, head crowning in
dilated cervix. ---Is just the skin on the top of the head a person?
Organism, cephalic presentation, station +3, uterus contracting, but
cervix not dilated yet ---This is probably clearly a fetus......**But the question remains: *
At what exact moment in the journey between clearly-a-fetus and clearly-a-neonate did it change from non-person to person?
What about cesarean section?
Before incision, during incision, after incision but before removing organism, or somewhere in the middle of the motion of removing the organism?
What about the gestational age at delivery?
A neonate 3 months preterm is born on the same day as a full-term neonate. They have the same birthday, but are at very different developmental stages - 6 months versus 9 months since the zygote stage.
- Is that preterm neonate still defined as a fetus? It is only at 6 months gestational age, and the full-term neonate was still defined as a fetus at 6 months.
- Is the preterm neonate a person yet, like the full-tern neonate is? Or
does it have to wait 3 months before it becomes a person?
Why go through this exercise?
This exercise basically asks the question "in the process of that human organism traveling from her uterus, through her vagina, and into the outside environment and severing the cord - when did it become a person?"
This is to illustrate that there is no good boundary. There is a long transition from being 100% in the uterus to 100% out of the vagina; there isn't a toggle switch that flips from unborn to born.
Most of these intermediate options seem ridiculous, but in order to logically say that "birth defines personhood," one of these intermediate stages has to be chosen as the distinction.
This distinction becomes critical in law, because the precise moment of binary change must be clearly defined in order to determine at what point ending the life processes of a human organism qualifies as "killing a person." Intentional infanticide is homicide. Murder. This includes neonaticide - killing within the first 24 hours.
If we leave those hours and inches of birthing undefined, any events occurring in that time will also be undefined. That lack of definition causes inconsistencies in policy and legality.
Then if we rely on our judicial system to create the life/not-life distinction for us, establishing precedents with each case it reviews, the criteria defining a living member of our species will be dependent on the current views of whomever holds political power at the moment.
An additional note about lacking boundaries: there is much debate in pro-choice circles as to what gestational age limit should be placed on abortions. This "intermediate-options exercise" should be absolutely critical to the success of these discussions - because in order to logically define life or personhood at birth, and also support partial-birth abortion or late-term abortion, one would be required to choose where along the continuum we explored is the absolute "defining moment." But I have rarely seen this discussed.
If we can't precisely define the distinguishing moment of change, we cannot establish solid criteria for life or personhood; therefore, the argument for personhood/non-personhood falls apart.
The only logical binary toggle-switch for the definition of "life" or "person" is the initial stage, when the individual organism first comes into existence - with its own unique DNA and capacity to complete human development. Prior to the formation of a zygote, there are two parental gamete cells with different haploid DNA, both incapable of development independently. That is a distinct biological change.
Rejecting the existence of any other boundary (based upon lack of scientific criteria to establish the distinguishing boundary) may very well be preferable to having a multitude of different ambiguous, poorly defined boundaries.
After having studied embryology, physiology, and aided in numerous vaginal and cesarean deliveries, I can honestly state that none of the intermediate options make sense to me.
A zygote will on its own develop into an adult, as long as nutrients and supportive environment are provided. Remember the placenta and amnion are 100% fetal tissue; the maternal organism supplies nutrients and environment only. An embryo will die without the uterus environment, just as an infant will die without other people taking care of it and feeding it.
Development is true a continuum; there is no moment at which the "life" or "personhood" toggle switch can be defined. Thus the distinction cannot exist.
Therefore, I consider human life to be a scientific definition:
A human organism is a living organism that develops from a single-celled stage throughout its natural life cycle into adult and senescence, when its life processes cease and it dies.
Living human organism = human life = "person."