Some will say that there is no human being until “sometime during infancy,” since that is the first point at which there is self-consciousness. But adults can lose consciousness--not to mention self-consciousness--and we don’t say they are no longer human beings.
Others will say that we have a human being “once birth takes place,” since at that point the baby is no longer connected to and totally dependent on something or someone else. But adults can become connected to and totally dependent on a ventilator to breathe for them in the same way--and we don’t say theyare no longer human beings.
Still others will say that we become human beings “before birth, at the point of viability,” since that by definition is when the fetus is capable of living independently should birth take place. But conjoined twins or adults dependent on pacemakers are not capable of living independently in that way--yet we don’t say they are not human beings.
Many have said that a human being is present at the point “when the fetus shows movement,” traditionally measured by when the mother feels the fetus move. But adults can become paralyzed, and we don’t say they are no longer human beings.
More recently, some have said that we have a human being about six weeks after fertilization, “when brain activity is detectable,” since that is when the potential for self -consciousness is present. But when adults lose consciousness, they can also lose the potential for self-consciousness--yet we don’t say they are no longer human beings.
Even more recently, others have begun saying that a human being is first present “when the embryo attaches to the wall of a mother’s uterus” (the point of implantation,” since without that attachment the embryo canot live. But when adults lose the capacity to breathe on their own and are in desperate need of being attached to a ventilator in order to live, we don’t say they are no longer human beings until they are attached.
In other words, there is no point later than fertilization after which an embro, fetus, or child gains something that an adult has, which is essential in order to be a human being. There is no capability or potential that makes something more definitively a human being than what genetics has already established at fertilization.