## Tuesday, May 30, 2017

### Killer arguments and the nature of proof in historical sciences

Some long time ago, somebody told me this joke, which I just found again on the internet in an English version (following jokes.cc.com, with modifications based on my memory):
Teacher: "Four crows are on the fence. The farmer shoots one. How many are left?"
Little Johnny: "None."
Teacher: "Listen carefully: Four crows are on the fence. The farmer shoots one. How many are left?"
Little Johnny: "None."
Teacher: "Can you explain that answer?"
Little Johnny: "One is shot, the others fly away. There are none left."
Teacher: "Well, that isn't the correct answer, but I like the way you think."
Little Johnny: "Teacher, can I ask a question?"
Teacher: "Sure."
Little Johnny: "There are three women in the park. The first one reads a love novel, the second one reads the newspaper, and the third one updates her FaceBook profile, which one of them is married?"
Teacher: "The one reading the newspaper?"
Little Johnny: "No. The one with the wedding ring on, but I like the way you think."
Given the title of this post, you may wonder why I tell you that joke. The reason is that for me, the essence of the joke is expressing the situation we often have in the historical sciences when we talk about "proof", be it of the closer relationship of different species, or the ultimate relationship of languages. Given the evidence we are given, we can reach an awful lot of conclusions in order to arrive at a convincing story, but if we see the wedding ring on somebody's hand, we know the true story no matter what other evidence we are given. The wedding ring in the joke serves as a killer argument — no matter what other evidence we consider, it is much more likely that the person who is married is the one with the ring than anybody else.

We often face similar situations in the historical sciences where we seek some kind of true story behind a couple of facts, when we are given external evidence that is just pointing to the right answer, or — let's be careful — the most probable answer, independent of where the other evidence might point to. We can think of similar situations in crime investigations, where we may think that a large body of evidence convicts some person as a murderer until we see some video proof that reveals the real offender.

That crime investigations have a lot in common with research in the historical sciences has been noted before by many people, notably the famous Umberto Eco (1932-2016), who edited a whole anthology on the role of circumstantial evidence in linguistics, semiotics, and philosophy (Eco and Sebeok 1983) where scholars compared the work of Sherlock Holmes with the work of people in the historical sciences. What Sherlock Holmes and historical linguists (and also evolutionary biologists) have in common is the use of abduction as their fundamental mode of reasoning. The term itself goes back to Charles Sanders Peirce (1839-1914), who distinguished it from deduction and induction:
Accepting the conclusion that an explanation is needed when facts contrary to what we should expect emerge, it follows that the explanation must be such a proposition as would lead to the prediction of the observed facts, either as necessary consequences or at least as very probable under the circumstances. A hypothesis then, has to be adopted, which is likely in itself, and renders the facts likely. This step of adopting a hypothesis as being suggested by the facts, is what I call abduction. I reckon it as a form of inference, however problematical the hypothesis may be held. (Peirce 1931/1958: 7.202)
Our problem in the historical sciences is that we are searching an original situation: what was the case a long time ago, based on general knowledge about (evolutionary or historical) processes and the results of this situation. When Sherlock Holmes looks at a crime scene, he sees the results of an action and uses his knowledge of human behaviour to find the one who was responsible for the crime. When doctors listen to the heartbeat of patients who are short of breath, they try to find out what causes their disease by making use of their knowledge about symptoms and the diseases that could have caused them. When linguists look at words from different languages, they make use of their knowledge of processes of language change and language contact in order to work out why those languages are so similar.

As do medical practitioners or crime investigators, we have our general schema, our protocol, which we use to carry out our investigations. Biologists search for similar DNA sequences, linguists look for similar sound sequences. In most cases, this works fine, although we are usually left with uncertainties and things that do not really seem to add up. As long as we can quietly follow the protocol, we are fine; and even if the results of our research do not necessarily last for a long time, being superceded by more recent research, we usually have the impression that we did the best we could, given the complex circumstances with their complex circumstancial evidence. But once in a while, we uncover evidence similar to video proofs in crime investigation, or wedding rings as in the Little Johnny joke — evidence that is so striking that we have to put our protocol to one side and just accept that there is only one solution, no matter what the rest of our evidence or our protocol might point to.

In 1879, Ferdinand de Saussure (1857-1913) predicted two consonantal sounds in Proto-Indo-European based on circumstantial evidence (Saussure 1879). In 1927, Jerzy Kuryłowicz (1895-1978) could show that one of the sounds was still pronounced in Hittite, an Indo-European language that was not known during Saussure's time (Lehmann 1992: 33), and had just been deciphered. While Saussure followed protocol in his investigation, Kuryłowicz provided the video proof, and only since then, Saussure's hypothesis has become communis opinio in historical linguistics.

I assume that nobody will doubt the existence of different kinds of proof, different qualities of proof, in historical disciplines. If we are left with nothing else but our protocol, we can derive certain conclusions, but we can easily abandon our protocol once we have been presented with those killer arguments, that specific kind of proof that is so striking that we do not need to bother to have a look at any alternative facts again. I do not know of any similar examples in biology, but in linguistics (and in crime investigation, at least judging from the criminal novels I have read), it is obvious that our evidence cannot only be ranked, but that we also have a huge incline between the standard evidence we use to make most of our arguments and those killer arguments that are so striking that no doubt is left.

In the short story The Adventure of the Beryl Coronet, Sherlock Holmes says:
[When] you have excluded the impossible, whatever remains, however improbable, must be the truth.
But this is only partially true, as in Sherlock Holmes' cases the truth is usually (but not always!) presented in such a form that it does not leave any place for doubt. Sherlock Holmes is a genius at finding the wedding rings on the fingers of his witnesses. As historical scientists, we are often much less lucky, but probably also less talented than Mr. Holmes. We are thus left with the fundamental problem of not knowing how to find the killer evidence, or how to quantify the doubt in those cases where we just follow the general protocol of our discipline.

References
• Eco, U. and T. Sebeok (1983) The Sign of Three. Dupin, Holmes, Peirce. Indiana University Press: Bloomington.
• Lehmann, W. (1992) Historical linguistics. An Introduction. Routledge: London.
• Peirce, C. (1931/1958) Collected Papers of Charles Sanders Peirce. Harvard University Press: Cambridge, Mass.
• Saussure, F. (1879) Mémoire sur le système primitif des voyelles dans les langues indo-européennes. Teubner: Leipzig.