The Dangerous Mule

This essay is concerned with mules. No, really. It is about the sterile offspring of horses and donkeys. The "mule question" is a fascinating and queer puzzle that deeply worried Aristotle. It is also a problem that has been almost totally forgotten about since Darwin. Before Darwin, the mule presented the first real challenge to the Aristotelean doctrine of essences of biological organisms. Since Darwin, philosphers have mainly focussed on the challenge evolution presented. By looking at his response to the mule doctrine, one can see how Aristotle might have responded to Darwin had their millenia been reversed.

Aristotle's biological theory was essentialist. Each species, including humanity, had an essence to it that didn't change over time. This was relevant to biology, as it tells us what we are looking for in biological research. It was the universal truth that could be inferred from the particular evidence. It was relevant to logic, since it told us what was properly considered a subject and what was properly considered a predicate. The Greek phrase to ti en einai, translated "essence", literally means "what it was to be". In other words, when you ask me "what I am", "human" is the truest, most specific answer I can give you. Finally, it was relevant to ethics, since each member of a species should be judged relative to the ideal capacities of a member of that species. For instance, a human being who cannot fly is not defective, but a sparrow that cannot fly is.

The mule was a threat to this conception of biology. Aristotle needed to establish a criterion according to which two organisms could be considered a part of the same species. An Ethiopian and a Greek are members of the same species, but a squirrel and a chipmunk are not. He settled on the criterion that has largely stuck in biology up to the present day, that of viable reproducibility. That is, you could recognise something as a member of a species, because it was able to make more of that species when mixed with another of its species.

This was not a completely arbitrary criterion. Aside from being the most obvious solution, Aristotle was concerned about where the essence of the baby animal would come from, that is, its formal cause. "Chipmunkness" had to get into a baby chipmunk somehow, so he believed that it would need to be from another chipmunk. Therefore, reproducibility and esssences worked hand-in-hand. Reproducibility explained where the baby's essence came from, and the natural division of essences could be discerned through reproducibility.

Enter the mule. The mule threatened to blow the whole system a part. (I have no evidence for this, but I believe Asimov was aware of this, which is why "the Mule" is a "mutant" who threatens the systematic predictions for Foundation). The mule was a sterile hybrid of a donkey and a horse. Aristotle was stuck. Donkeys and horses can't reproduce together viably, since their offspring are sterile. Therefore, donkeys and horses are different species. However, if donkeys and horses are different species, then which species is the mule? It clearly has horse parts and donkey parts, so it is not really just a horse or just a donkey. However, there cannot be a separate species of "mule" either, as they cannot reproduce at all, let alone viably. So, the mule is not essentially a horse or a donkey or a mule. Apparently the mule, then, has no essence, that is, it has no to ti en einai, since it is of no particular species.

This can't be right. The mule is walking around and braying, so it must have an essence. Aristotle had two solutions for this. His first solution doesn't work. He suggests that the mule may be an instance of the genus of which horses and donkeys are members, for our purposes, the genus equus. This creates a serious problem, though. One can say that a horse is essentially a "equus caballus" and a donkey is essentially a "equus asinus". It is the "caballus" and the "asinus" that provide the essence, though, and properly answer the question of "what it is". "Equus" is just a category according to which we group species. If a mule is just an "equus", full stop, it has no species and no essence. Worse, Aristotle is claiming that, even though it is not a member of any species it is still somehow a member of a group of species, which makes even less sense.

The second solution Aristotle gives is that the mule is just an exception to the normal regularity of nature. Aristotle occasionally does this, and it seems as though he is just throwing up his hands. However, it is an important part of Aristotle's methodology. He says that it is shameful (aischron) to treat a subject with more precision than it deserves. One should not expect a system to fit nature precisely. The mule is a case of that.

This tells us something very interesting about Aristotle sees his system of biological essences. He believes that his account is only an approximation and is willing to countenance exceptions. How, then, might he respond to Darwinian evolutionary theory? Since Darwin, many people have argued that Aristotelean essences and biology have been refuted. One of the main reasons this claim has been made is because Darwin claimed that the line between species is not sharp but blurry. However, Aristotle had already made room for the possibility of blurry interspecies lines. His doctrine of essences does not require that there be no blurring between the species, only that a discernible population that can reproduce viably exists. This is still true, and nothing about evolutionary biology shows otherwise. As such, Darwinian biology does not refute Aristotle's theory of essences.

The mule posed a great challenge to Aristotle's theory of biology. Fortunately, the problem of mules gave Aristotle the opportunity to wrestle with the fluidity of species that would not seriously resurface for over two thousand years. Because of this opportunity, it is possible to have some sense of how Aristotle might have responded to evolutionary biology.