The question of when a person’s life begins is deeply complex. To approach this question, we first need to clarify what we mean by "person." Three definitions of person you may have heard before are “human being,” “human organism,” and “human organism with the ability to think thoughts and feel emotions.” In casual conversation, does it matter which of these definitions of person you use? Not really. But in formal conversation, where details are important, does it matter which definition of person you use? Absolutely.

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"human being"

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If you heard, “Aunt Sue is a human being. She deserves to be treated with respect,” you’d probably nod along in agreement. In casual conversation, emphasizing that someone is a “human being” is a way to highlight their importance. But is “human being” a good formal definition of person? Surprisingly, no. When we apply logical syllogism to this definition, we uncover a problem: all living human cells, from skin cells to muscle cells to bone cells, etcetera, are technically “human beings.” Using the definition “human being” produces the technical result that a single human skin cell deserves the same respect as Aunt Sue.

 

The syllogism goes like this:

 

All living cells are living things.

All living things are beings.

Therefore, all living cells are beings.

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Then we apply human categorization, and we can see:

 

All living human cells are human beings.

 

If we formally use “human being” as our definition of person, the result is that we formally grant personhood to all living human cells, from skin cells to muscle cells to bone cells, etcetera.

 

This clearly isn’t what we intended to happen from our definition of person, so “human being” is not an appropriate formal definition.

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"human organism"​

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Next, let’s look at “human organism.” On the plus side, individual human somatic cells are not organisms, so this definition solves the problem of the first. On the negative side, this definition is problematic, too, when we apply deductive reasoning and modus tollens logic.

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Modus tollens works like this:

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If P, then Q.

Not Q.

Therefore, not P.​

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If an "If P, then Q" statement is true, then according to modus tollens the statement "If not Q, then not P" must also be true.

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Let's look at the following "If P, then Q" statement:

 

"If we face a scenario where we can save either a large group of people or a small group of people and there are no extraneous variables differentiating the groups [P], then we save the larger group [Q]."

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This statement is always true when 1) there truly are no extraneous variables that skew our choice toward the smaller group and 2) the individuals in the groups are truly persons.

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Before we continue, some examples of extraneous variables to watch out for include but are not limited to:

• Poor health: skews the result toward saving the healthier group

• Ageism: when there is an age gap larger than a couple of years, it skews the result toward saving the group of people who have more years left to live

• Intelligence/talents: skews the results toward saving the group that has more intelligence/talents to offer society

• Established relationships: skews the results toward saving the group that the rescuer feels more positively related to

• End of the world scenario: skews the results towards saving the group that is more useful in stopping the end of the world because without the world everyone ends up dead anyway​

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Now let's use this knowledge to test the "human organism" definition of person. Two terms that fit this definition are zygote and newborn. And to be clear, these terms imply less than a year of age difference, which wards off any serious concern of ageism creating a preference for the zygotes. Using these specific terms does not add extraneous variable(s) to the scenario, so we can use these terms to test the "human organism" definition with modus tollens:

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If we have a tragic scenario where we can save either a large group of zygotes or a small group of newborns, then which group do we save?

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Everyone saves the small group of newborns.

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Modus tollens proves that since we did NOT "save the larger group," then something in P must be false.

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In testing the "human organism" definition of person, P assumes that zygotes are people and P assumes that newborns are people. 

 

Since P is false, then at least one of those assumptions must be false.

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Newborns are people, so the assumption that newborns are people is correct.

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This leaves us with only one possibility: Zygotes are not people.

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And, in effect, "human organism" is not the definition of person.​ Being a person requires more than being a human organism. Newborns have it. Zygotes don't. But what is "it"? And at what developmental stage does "it" arise?

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At first, some people may guess the "it" is heartbeats, but that would leave people suffering from cardiac arrest left out from being considered persons. Other people guess the "it" is reflexes, referencing the quickening, which is the point when a pregnant woman can feel fetal movement in her womb, but this doesn't work because dead bodies can have reflexes, too. Another guess for "it" is viability, the ability of a living thing to survive. Consider examining a living thing's viability, but not already knowing whether or not the living thing is a person. Does the fact that a living thing is deemed viable assure us that it is a person? No. Viability doesn’t tell us if a living thing is a person; therefore, it makes little sense to use viability in a definition for person.

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"human organism with the ability to think thoughts and feel emotions"​​​​​

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Let’s look at the last definition from the three mentioned at the beginning of this page, where a person is defined as a "human organism with the ability to think thoughts and feel emotions,” or more technically defined as a “human organism with a brain that contains the functional infrastructure necessary for the ability to think thoughts and feel emotions.”​

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It is important to note that at its core, the challenge of defining "person" is philosophical. Philosophy is the study of personhood, existentialism, logic, truth, and morality. Because philosophy is not commonly taught in schools, it's not surprising that many people mistakingly attempt to define "person" in purely biological terms (as a "human organism") without reference to personhood's philosophic roots ("cogito, ergo sum" and "tabula rasa"). This last definition, a "human organism with the ability to think thoughts and feel emotions" goes beyond the biological to include philosophical consideration.

 

Philosophically, personhood requires mental and moral capacities. If a human organism does not have a brain with the functional infrastructure necessary for the ability to think thoughts and feel emotions, then that human organism certainly does not possess the mental and moral capacities required to be a person. Therefore, this last definition can put into words why newborns are considered people while zygotes are not. Newborns have brains with the ability to think thoughts and feel emotions. Zygotes do not have brains.

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One important question is "Does this last definition include unconscious people?" The answer is yes. Unconscious people are included because there is a difference between having an ability and actively using that ability. For example, someone who has the ability to sing like an angel still has that ability even when they're simply relaxing on a couch and watching TV instead of singing. Another example is that you can think thoughts and feel emotions while dreaming, but while dreaming you aren't conscious. Technically speaking, however, consciousness isn't binary. It is not on (conscious) or off (unconscious) but rather exists as a spectrum from fully conscious (being fully aware) to semi-conscious (being very sleepy and not fully alert) to minimally conscious (asleep or in a coma, where one may have dreams or thoughts inside their head without being aware of the outside world). In a situation where there is absolutely zero consciousness, meaning no alertness, no dreams, and no brain activity of any kind, then the person is brain-dead, not unconscious. A brain-dead person is a dead person, even if parts of the remaining human organism are still alive and functional (this is how organ donation is possible). 

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Furthermore, there is also a difference between having an ability and having a memory of using that ability. Do we remember all the thoughts and emotions from when we were toddlers? No. But not remembering doesn’t negate the fact that toddlers think thoughts and feel emotions. This distinction affirms that people who suffer from dementia are, in fact, still people, too. The bottom line is that once a human organism develops the ability to think thoughts and feel emotions, the ability continues to exist until brain death.

 

Another question is, “What about thinking, feeling beings that aren’t human? Is it fair to deny them personhood just because they’re non-human?” Intelligent species like dolphins and elephants undoubtedly have the ability to think thoughts and feel emotions. They have their own personalities, too. Why do we deserve personhood and associated rights and yet they do not? Artificial Intelligence (AI) may one day have the ability to think thoughts and feel emotions. Would AI then not be a person? And what about the possibility of extraterrestrial lifeforms who have the ability to think thoughts and feel emotions? Would they not scoff at us, “Narcissistic humans, they used to believe the universe revolved around their planet, and they still believe that personhood revolves around their species!”

 

A central understanding of personhood is that being a person means being a part of a moral community, having the moral right of self-determination, and having the moral responsibility of treating all other persons respectfully. Can we trust thinking-feeling-non-humans to be a part of our moral community? Can a species that is not literate, that cannot communicate at a high enough level to understand laws and rights, be reasonably expected to bear the responsibility of treating all persons respectfully?

 

All thinking, feeling beings deserve to be treated equitably and respectfully by persons, but the question of non-human personhood is yet to be settled. In the spirit of not being close-minded, we will therefore emphasize that “a human organism with the ability to think thoughts and feel emotions” is a workable definition for “human person,” not “universal person.” We do not yet have a workable definition for “universal person.” Regarding our focus on settling the abortion debate in the United States, we only need the “human person” definition because the only people involved in this social reckoning are human people.

 

For human persons, the ability to think thoughts and feel emotions arises from within one specific organ: the brain. When your brain ceases to function—that is, when your brain ceases to have the ability to think thoughts and feel emotions—your life as a person ends. Likewise, when your brain gained the ability to think thoughts and feel emotions, your life as a person began. In order for the ability to think thoughts and feel emotions to exist, a minimum level of brain infrastructure must first exist.