How does the idea of natural law from Summa Theologica by Aquinas apply to abortion?
Summa Theologica by Thomas Aquinas does not directly apply the concept of natural law to the problem of abortion. As neither the Bible nor Aquinas prohibits or permits abortion explicitly, this question requires some complex analysis.
First, to analyze this, a student would need to review the concept of Natural Law as the human application of reason to law and morality. Because humans have a limited mortal intelligence, humans cannot directly grasp Eternal Law, but instead, use reason to approximate it through natural law. Although understanding of natural law may be imperfect, natural law itself is still universal and eternal, rather than contingent and conventional in the manner of human law.
The most important issue for applying natural law to abortion is the question of ensoulment. The notion of ensoulment, or the moment at which the soul descends into the body, is discussed in the Summa Theologica. While some Christian thinkers argues that this occurred and was signaled when a baby uttered its first word (a sign of the presence of the divine logos) others argues for it occurring at specific points in the development of the fetus and a small number argued that ensoulment began at conception (but this was a minority position and medieval technology would not have been able to confirm pregnancy until a fairly late stage).
Essentially, abortion that occurs after the instant of ensoulment would be a form of murder, violating natural law, but abortion (or even infanticide) before ensoulment would have been morally neutral, as organic matter without a soul is not human.
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