Michael Egnor: A common argument as to why abortion is generally ethical is that the unborn child cannot reason:
But human life is a continuum precisely because every stage of human life — from zygote to senescence — is human. A zygote is just as much a human being as you and I are. You and I were just as much human beings when we were zygotes as we are now, and we will be just as much human beings when we are on our deathbeds as we are now.
Children in the womb at every stage are certainly human beings, just as we all are. Human life is a developmental process of actualization of potencies. We grow, physically, mentally, and spiritually. No one is less than human just because of age or stage of development or ratios of potency to actuality.
We are all human beings at every stage of life. Whether we are “persons” or not depends on the reigning moral and legal definition of “persons.” The moral quandary — the agonizing and contentious moral quandary in the abortion debate — is not whether zygotes or embryos or fetuses or newborns are human beings (they are), but whether they are persons worthy of respect and protection.Michael Egnor, “Must we be able to reason to be thought of as human persons?” at Mind Matters News (May 15, 2022)
Takehome: Reason is a natural quality of the human being but, like everything in nature, it develops in stages. One can’t say it doesn’t exist when undeveloped.
You may also wish to read: Do babies really feel pain before they are self-aware? Michael Egnor discusses the fact that the thalamus, deep in the brain, creates pain. The cortex moderates it. Thus, juveniles may suffer more. Jonathan Wells recalls, from when he was a lab technologist, how very premature infants would scream when he took a drop of blood for tests.