Patterns, processes, abduction, and consilience

In a recent blog post, David emphasized how important it is to distinguish patterns from processes in evolutionary biology, with phylogenetic analysis concentrating on the description of patterns (and not on the direct investigation of processes. David's major point is that we need to be careful to not forget about the logical limitations of our approaches:

In the world of logic, propositions cannot be converted; and yet converting propositions is exactly what is done by all descriptive data analyses.

As David correctly points out, in phylogenetic analysis, we tend to observe a pattern (some similarity between different species or languages, for example), and use this pattern to conclude that a specific process has happened (eg. the languages are so similar that we think they are identical).

Given that this problem is also important in historical linguistics, I want to share some thoughts from a linguistic perspective. Most of these were elaborated much earlier, in my PhD dissertation; and if you have read the original chapter (List 2014: 51-57), what I write below may seem repetitive. I have also alluded to these ideas in a couple of previous posts: What we know, what we know we can know, and what we know we cannot know; and Killer arguments and the nature of proof in historical sciences.

However, it is worthwhile to elaborate on these thoughts here, as David's comments are extremely interesting for historical sciences in general, and I think they deserve a more proper discussion across different disciplines.

Ontological fact and epistemological reality

The basic pattern/process problem may be even more complex than it is in evolutionary biology. In quite a few branches of science, most prominently in the historical and social sciences, even the object of investigation is not directly accessible to the researcher. All researchers can do is to try to infer the research object with the help of tests. In historiography we infer the res gestae by comparing direct and indirect (usually written) sources (Schmitter 1982: 55f). In psychology, attributes of people, such as "intelligence" cannot (yet) be directly measured but have to be inferred by measuring how they are "reflected in test performance" (Cronbach and Meehl 1955: 178).

The same holds for ancestors in historical linguistics and evolutionary biology. All we can do in order to examine whether some languages or species share a specific kind of ancestry is comparing them systematically, trying to identify patterns that provide evidence for close relationship. Given that we lack direct evidence of its existence, the ancestral languages or species we infer through comparison cannot be treated like an ontological fact but only as an epistemological reality (Kormišin 1988: 92). We address what psychologists call the construct, that is, the "fiction or story put forward by a theorist to make sense of a phenomenon" (Statt 1981/1998), not the "real" object.

Abduction as our sole mode of logical reasoning

In historical linguistics, we can address our research objects only via constructs, and so we have to rely on abduction as our sole mode of logical reasoning (Anttila 1972: 196f). The term abduction was originally coined by Charles Sanders Peirce (1839-1914) and refers, as opposed to induction and deduction, to a "mode of reasoning [...] in which rather than progressing 'logically' [...], one infers an antecedent condition by heuristic guessing from a present case" (Lass 1997: 334). In Peirce's word:

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)

Due to the specific aspects of knowledge we are given in the historical sciences, abduction is the only mode of reasoning that we can employ. According to Peirce (ibid.: 2.623), all three modes of reasoning, induction, deduction, and abduction, "involve the triad of 'rule', 'case' and 'result', but inference moves in different directions" (Lass 1997: 334). While induction infers a rule from a situation in which one is given case (initial situation) and result, deduction infers a result from a situation in which one is given case and a rule. Abduction, however, starts from a result (or a pattern in David's words) and a rule from which we try to infer a case.

As an example, consider the problem of language evolution. Given two languages with no written records of their previous history, we may observe as a pattern (or result) that they show striking systematic regularities in terms of sound correspondences. Given that we know, that — as a rule — languages change their sound systems slowly over time, we can conclude that the initial situation, the case, was that the two languages were once a single language. There is no way we employ any other mode of reasoning here, as long as we start from individual languages (or species) whose past we want to understand and describe.

We can think of situations in which we try to induce rules in historical linguistics, for example, when dealing with the development from Latin into its descendant languages, where we could ask about the individual processes of sound change (or sound change rules) by which the former was transformed into the latter. We can also think of situations in which we try to decide results from rules and initial situations, for example when trying to predict unobserved cognate words in languages that have not yet been completely documented by fieldwork (Bodt et al. 2018), by applying rules of sound change (or sound correspondences) to aligned cognate sets (List, forthcoming). But the big bulk of our work in historical linguistics (and also in evolutionary biology) works only via abduction: given a result (a pattern / observation in the present), we use our knowledge of rules and processes to infer an ancestral state.

Problems of reasoning based on abduction

According to Schurz (2008), different patterns of abduction can be distinguished, depending on: (1) "the kind of hypothesis which is abduced", (2) "the kind of evidence which the abduction intends to explain", and (3) "the beliefs or cognitive mechanisms which drive the abduction" (ibid.: 205). The kind of abduction that is commonly used in historical linguistics and evolutionary biology belongs to the family of factual abductions, that is, abductions in which "both the evidence to be explained and the abduced hypothesis are singular facts" (ibid.: 206). Since we mainly deal with unobservable facts (ie. constructs), we can further characterize it as historical-fact abduction (ibid.: 209).

The problem of historical-fact abduction is not necessarily that what we are try to "observe" lies in the past, but more importantly, that — due to the logic underlying abduction as a mode of reasoning — we usually have to infer both the rules and the initial situation from the patterns we observe. Given (as David emphasized) that a pattern can result from different processes, our inference of a specific, individual historical fact requires that we decide on a specific, individual process at the same time. Given that we have to infer both the process and initial state at the same time, it is not surprising that our inferences about the past are often so vague, and may easily change so quickly, specifically in a situation where we can't just travel back in time to see whether we were right.

In contrast to David, who suggested that we cannot directly investigate processes in the evolutionary sciences, however, I think that in we still can indirectly, be it with help of experiments, with simulations, or in those cases where we are lucky enough to find history documented in sources. These cases where we can study processes, however, are — and here I agree completely with David — not what we normally do in our research. What we usually do is investigating patterns and trying to infer both the process and the original state by which the patterns can be explained.

Cumulative evidence

The problem of abduction, in general (or historical-fact abduction, in specific), is to make sure that we protect ourselves from giving in to wild speculations. That we are not necessarily good at doing so is reflected in the numerous debates in historical linguistics, and evolutionary biology, where scholars at times invoke completely contrary scenarios explaining the past based on identical patterns. In addition, in historical linguistics, people often do not even agree regarding the patterns they believe can be observed in the data.

Earlier, in my dissertation (List 2014), I identified two aspects that I deem important in order to minimize the speculative aspect of our research, claiming that historical-fact abduction should be based on: (1) unique hypotheses, and (2) cumulative evidence. That we need unique hypotheses may seem self-evident at first sight, since it seems to be silly to claim that a certain pattern could be explained by a range of processes. Looking back at this point now, however, I tend to see this less strictly. In fact, I think that I would even prefer it if scholars would list all potential (individual) processes that may seem likely to have yielded a pattern, instead of focusing only on one possibility (and disregarding alternative solutions). Since we are not doctors who need to heal our patients as quickly as possible, we can afford a certain amount of doubt in our research.

Regarding the second point, what I had in mind earlier was that it is best if we have multiple results or different patterns (observed for the same species or languages under investigation) that can all be explained by the same hypothesis. In order to justify the claim that one specific hypothesis explains the evidence better than any alternative hypotheses, we can profit from combining multiple pieces of evidence that might "[fall] short of proof [when taking] each item separately" but become convincing when "all the items [are] combined" (Sturtevant 1920: 11).

Being forced to rely on multiple pieces of evidence (that only when taken together allow one to draw a rather convincing picture of the past) is not a unique problem of historical linguistics and evolutionary biology, but also of historiography – and even crime investigations, as was pointed out by Georg von der Gabelentz (1840-1893, cf. Gabelentz 1891: 154), and in later work on semiotics (cf. the papers in Eco and Sebeok 1983). The fact that historical linguistics theories are built about cases (events, unique objects), as opposed to theories about general laws, may also be the reason for the philological "style" prevalent in historical linguistic studies. I also believe that it is due to the complex nature of the inference process that a systematization of our methods has never been carried out efficiently.

While, for example, we can claim (at least to some degree) that the identification of cognate words in historical linguistics can be systematized (and even to some extent automatized, List et al. 2017), we are at a loss when it comes to systematizing the methods that we use to determine whether words have been borrowed or not. Instead of using one single method, we use a whole range of indicators, and only take borrowings for granted if at least a few of them point into the same direction (List 2018).

Consilience and conclusion

In a talk by James McInerney, held in 2015 in Paris (presenting an overview of his research as reflected in part in McInerney et al. 2014), I realized that the question of "cumulative evidence", which I had thought would have been discussed only in linguistic circles, belongs to a larger complex of discussions about consilience, as opposed to the Popperian tradition that claims that knowledge in science can only advance via falsification and the identification of general laws, as opposed to singular facts (Popper 1945: Chapter 25:II). We find this view, that we need to employ cumulative evidence when trying to infer individual facts, clearly stated in the work of William Whewell (1794-1866), who originally introduced the term consilience:

The Consilience of Inductions [ie. abductions] takes place when an Induction obtained from one class of facts, coincides with an Induction, obtained from another different class. This Consilience is a test of the truth of the Theory in which it occurs. (Whewell 1840: 469)

As far as I understand from James McInerney's talk, the idea of consilience has long been disregarded in the historical sciences but is now gaining popularity (also thanks to the influential book by Wilson 1998). Although at first I felt delighted when I realized that I was not alone with the problem that I had called "cumulative evidence", based on the old book by Sturtevant (1920), I have to admit that I still do not really know what to do with this information, as it is extremely hard to operationalize the concept of consilience. When confronted with numerous different pieces of evidence, how can we identify the hypothesis that explains them all? How can we compare two opposing hypotheses that each convincingly explain some but not all the data? How can we arrive at an objective weighting of our evidence, based on its importance?

What is clear to me is that a "probabilistic evaluation of causes and elimination of implausible causes plays a central role in factual abductions" (Schurz 2008: 207), since it reduces the search space when seeking an explanation for a given phenomenon (ibid.: 210f). But it is not clear how to arrive at such an evaluation when dealing with patterns in practice. For the time being, thinking and discussing about consilience seems interesting; but until we find ways to operationalize it, it will just remain a nice idea without any concrete value for our scientific endeavors. I dearly hope that this won't be the case.

References

Anttila, R. (1972) An introduction to historical and comparative linguistics. Macmillan: New York.

Bodt, T., N. Hill, and J.-M. List (2018) Prediction experiment for missing words in Kho-Bwa language data. Open Science Framework Preregistrations .evcbp., 7 pp. [Preprint, under review, not peer-reviewed]

Cronbach, L. and P. Meehl (1955) Construct validity in psychological tests. Psychological Bulletin 52: 281-302.

Eco, U. and T. Sebeok (1983) The sign of three. Indiana University Press: Bloomington.

Gabelentz, H. (1891) Die Sprachwissenschaft. Ihre Aufgaben, Methoden und bisherigen Ergebnisse. T.O. Weigel: Leipzig.

Kormišin, I. (1988) Prajazyk. Bližnjaja i dal’njaja rekonstrukcija [The proto-language. Narrow and distant reconstruction]. In: Gadžieva, N. (ed.) Sravnitel’no-istoričeskoe izučenie jazykov raznych semejTeorija lingvističeskoj rekonstrukcii [Theory of linguistic reconstruction]. 3. Nauka: Moscow, pp. 90-105.

Lass, R. (1997) Historical linguistics and language change. Cambridge University Press: Cambridge.

List, J.-M. (2014) Sequence comparison in historical linguistics. Düsseldorf University Press: Düsseldorf.

List, J.-M., S. Greenhill, and R. Gray (2017) The potential of automatic word comparison for historical linguistics. PLOS One 12: 1-18.

List, J.-M. (2018) Automatic methods for the investigation of language contact situations. [Preprint, under review, not peer-reviewed]. URL: http://lingulist.de/documents/papers/list-2018-automatic-methods-for-the-investigation-of-language-contact.pdf

List, J.-M. (forthcoming) Automatic inference of sound correspondence patterns across multiple languages. Computational Linguistics 45: 1-24.

McInerney, J., M. O’Connell, and D. Pisani (2014) The hybrid nature of the Eukaryota and a consilient view of life on Earth. Nature Reviews Microbiology 12: 449-455.

Peirce, C. (1931/1958) The collected papers of Charles Sanders Peirce. Harvard University Press: Cambridge, Mass.

Popper, K. (1945) The open society and its enemies. Routledge: London.

Schmitter, P. (1982) Untersuchungen zur Historiographie der Linguistik. Struktur — Methodik — theoretische Fundierung. Gunter Narr: Tübingen.

Schurz, G. (2008) Patterns of abduction. Synthese 164: 201-234.

Statt, D. (1998) Consice dictionary of psychology. Routledge: London and New York.

Sturtevant, E. (1920) The pronunciation of Greek and Latin. University of Chicago Press: Chicago.

Whewell, W. (1847) The philosophy of the inductive sciences, founded upon their history. John W. Parker: London.

Wilson, E. (1998) Consilience: the unity of knowledge. Vintage Books: New York.