Imagine Hamish McDonald, a Scotsman, sitting down with his Press and Journal and seeing an article about how the "Brighton Sex Maniac Strikes Again." Hamish is shocked and declares that "No Scotsman would do such a thing." The next day he sits down to read his Press and Journal again and this time finds an article about an Aberdeen man whose brutal actions make the Brighton sex maniac seem almost gentlemanly. This fact shows that Hamish was wrong in his opinion but is he going to admit this? Not likely. This time he says, "No true Scotsman would do such a thing."
-- Anthony Flew, Thinking about Thinking, Or Do I Sincerely Want to Be Right, 1975.
Aristotle's biology identified living things--all of them-- as having a kind of soul as their form or essence. There was a hierarchy of such essences, classified by the soul's capacities. Plants possessed a nutritive soul, once which could consume, grow, and reproduce. Animals had these abilities, plus a sense of their surroundings and ability to move in response to their surroundings which plants seemed to lack: As such, they had an animal or "sensitive soul," which in addition to nutritive function had sensation and movement. Finally, humans were unique in having a "rational soul" because they can reason.
Modern biology categorizes life into kingdoms, some of which, like fungi, Aristotle might have considered to have a "nutritive soul." In addition, modern biology includes an extensive body of knowledge about microscopic life forms about which very little was known prior to the seventeenth century. Such life forms often have characteristics which are neither plant nor animal. For example, unlike macroscopic plants and animals, such cells may not undergo the cyclical larval and adult growth and development seen in the life cycle of typical animals, but instead may simply split at a particular size or age into two cells in a process called reproduction by mitosis. Certain species of bacteria may be motile or entirely non-motile in their normal colony formations (see here for example), so that presence or lack of motility may overlap both plants and animals within a fully normal variety.
To avoid controversy and the category errors that might otherwise be made about such microorganisms, biologists have chosen to place the archaea and the bacteria in their own classification domains, entirely separate from the hierarchy into which Aristotle classified the souls of plants and animals.
The "no true Scotsman fallacy," as described by Flew at the top of this posting, rests on a purported disagreement about the essence of a true Scotsman. Hamish is certain that certain standards of behavior are essential to being a Scotsman, and when confronted with a counterexample to his belief, he retreats in argument to suggesting that the exception is an aberration from what the true essence must be.
Consider now the possible plight of a biologist discussing the classification of bacteria with a classicist who wishes to determine whether a particular bacterium has a plant or and animal soul? The dialog might go this way:
C: So a spirochete is an animal. Look how it evades attack by white cells by moving away from them. It's alive and responds to its environment by moving from place to place. So it has a sensitive, animal soul.
B: Well, actually, spirochetes are bacteria. They are neither plants not animals, since they lack things like membrane-bound organelles such as mitochondria or a nucleus, and their DNA lacks histones and is structured differently from that of animals. So even though they may be motile, that does not make them true animals.
C: This sounds like the true Scotsman fallacy. I know an animal when I see one. What justifies using little differences like those to split one kind of animal off from others, as not being an animal at all?
B: Let's pretend that on a faraway asteroid there is a group of mining robots, built long ago by a group of nanotech-savvy aliens. The bots can grow and repair themselves and can reproduce themselves. They respond to the asteroid environment by mining it, and they can move around on its surface freely. They are not conscious, but function well at mining. So, they must, I think, have animal souls?
C: No, because they would be machines. They are not alive. Aristotle was classifying life, not tech. They don't have souls.
B: How is what you are saying different from my telling you that bacteria and archaea are neither plants nor animals, but an entirely different category of life than the ones Aristotle wrote about? Who is to say Aquinas would not agree that bacteria lack animal souls?
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In the true Scotsman story, Hamish has an ulterior motive in saying the Aberdeen evildoer cannot be a true Scotsman: he wishes to maintain a high opinion of the morals of his own ethnic group. Similarly, the Thomist has an ulterior reason to maintain the primacy of Aristotle's classification system for species: Aquinas identified the Christian concept of the immortal soul with Aristotle's rational soul, the form of the human body. Since Thomism identifies the Christian soul with a theoretical construct of Aristotle's biology, Aristotle's biology becomes a part of Aquinian theology.
Where does the issue of vagueness in the biology of species come up here? Vagueness in deciding just what a species is or exactly which organisms are that species might mean that it's vague whether or not a particular animal has a particular form or essence. Aquinas' essentialism allows each living thing to have only one essence, and so if what essence a living thing has is vague, just what single essence it has may also be uncertain and subject to error. Would this mean that whether a given person-- say, a premature infant-- had a human soul, or was in fact truly human, was also subject to error? Vagueness in biology should not lead to vagueness in ethics, should it?
Scotus' view of human biology and psychology was more nuanced and less monolithic than Aquinas', as it conceived of living things to be composed of many modular essences united by a unifying principle--a principle we might now call its DNA. His ideas about natural kinds, it seems to me, have much in common with today's medical concept of the human body as a composite of many separate yet intertwined systems and organs. A more nuanced, combinatorial view of essences allows room for vagueness. Too bad that instead a monolithic, unitary "just one form for each living body, and nothing else in a living body has any form" Aristotelian doctrine held sway in medieval science for so many centuries.
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