The Ontology of Abortion: Categories and essential properties

Can we draw a line between persons and nonpersons? Or between anything and anything else?

The abortion debate is still shaped by the assumptions we have inherited from Aristotle. This is not just true of people who have deliberately studied him, such as Catholic students of Aquinas. It is even more true of people who have never read or even heard of Aristotle, because it never occurs to them to think any other way.

Aristotle taught us to believe that the world consists of items which can be grouped into categories. The borders of these categories are determined by enumerating the essential properties that all members of a category possess. It is possible to make logical inferences about individuals by citing those essential properties as proof that they belong to a category. Aristotle’s favorite example was all men are mortal, Socrates is a man, therefore Socrates is mortal. In order for this kind of logic to be completely reliable, we have to accept what Aristotle called the law of the excluded middle. This principle rejects the possibility of blurry lines between categories. Everything in the universe is either a mammal or not a mammal, either a bagel or not a bagel, either a vegetable or not a vegetable.

Many anti-choice advocates rely on applying this principle to the category of human being. They insist that there must be a sharp line between humans and non-humans, and between one human being…