Taxonomies and ‘the Reichenbach rule’

This post is more for informational purposes and not so much making my mind up about something. Lately I’ve come across differing notions in the taxonomical organisation of life forms in contemporamous biology, as opposed to the Early Modern Period which I normally make reference to. The role of taxonomy is an issue which relates to an aspect of Immanuel Kant’s philosophy of science which I see having a big importance. I will consider a specific case of dispute in biology, brought to my attention by Samir Okasha (2002).

While virtually all biologists maintain the veritude of a system of ordering life forms. I say life forms as the most general taxon term to include as robust a set from viruses to bananas. What some quibble over, however, is the ordering system of these life forms. In a contemporary context, there are differing notions of classifying species. One concerns addressing ancestry, and so a life form is ordered in a taxonomical structure in relation to its ancestral species. Cladistics is the approach which looks at organisms by organisation of their ancestors and descendants, and nothing more. This may allow for a family or ‘clade’ of species to emerge from any given ancestor. Cladistics can be distinguished from earlier ‘Linnaean’ approaches which emphasis specific classification schemes like ‘phylum’ and ‘genus’, in order to reflect the complexity of genealogy. It is said that Cladistics is a more parsimonious approach to the traditional Linnaeus taxonomical scheme, because taxa are used sparingly in the former approach by its appeal to ancestry.

By contrast there is another notion of taxonomy: Evolutionary systematics. Systematics is an approach which varies from Cladistics in that it identifies and organises organisms not by ancestry, but by accounting for evolutionary heritage (let’s call this vertical ordering), as well as preserving a form of ‘horizonal’ classes of species akin to the traditional Linnaean approach. This approach varies in its emphasis on trying to merge the 18thC Linnaean scheme, with the 19thC notion of natural selection. Instead of looking at individual organisms, the focus for systematics are the emergence of groups of species, such as dinosaurs, and their descendants. In some respects the approach of systematics allows for more flexibility in the organisational scheme, and is less rigorous.

These two approaches of systematics and cladistics vary in the way that they cut across organisms to create taxonomies, to invoke a philosophical phrase, they quibble on how they ‘carve nature at its joints’. It gets philosophical when we consider the role of the notion of a ‘common ancestor’. A taxonomical scheme can be said to be monophyletic if there is a common ancestor to a group. The two approaches construe the notion of of common ancestry in different ways. Systematics approaches hold that a grouping has a similar ancestor, while Cladistics holds that the whole ancestral group is a common ancestor of a given group.

Why is this important for someone thinking about Kant?

This is potentially an example of how systematicity, a dictum about the nature of science, actually works in practice. The one thing that is not disputed in any way, is that there needs to be a taxonomical system. New evidence, and differing approaches allow for the system to be refined, expanded and even significantly re-ordered. Yet, the idea of Kant’s system is distinctly a priori. The potential area that needs more work in my view, if the claim that biological taxonomy is an actual example of Kantian Systematicity, is the role of the monophyletic, or the common ancestor. in Kant’s view, the ‘higher genus’ concept required that in principle there is a single highest taxon or entity to give rise to its ‘lower’ concepts. I used to think that this meant an a priori claim that there was a potential single taxon at the highest level, even if it is not discovered.

The case of Reichenbach’s critique of Kant’s metaphysics of space and time come relevant here. Kant was criticised severely on the basis that his rational geometry was fundamentally Euclidean, which was the basis of his metaphysics and epistemology of time and space. Spacetime, as the physicists have come to know it, is a much more complicated affair than Kant had come to know it the 18thC. If a philosophical theory makes a claim that is subject to empirical review, then the evidence of whether the claim stands up to empirical scrutiny makes the theory stand or fall, on the basis of the evidence. I shall call this the Reichenbach rule. To what extent is there an issue of Kant violating ‘the Reichenbach rule’ in Kant’s theory of systematicity in relation to the issue of taxonomy. I think that it relies heavily on what the taxonomical approaches hold in relation to the notion of the common ancestor, and how it is possible to interpret biological taxonomy in terms of systematicity.There are other issues as well as the Reichenbach rule, which also weighs of importance to compare, namely: whether a theory can change its components and structuring, while maintaining some aspect of truth to it, or the a priori ‘necessity’ of structure and also whether the fact ‘that there is a system’ is preserved through the differing contemporary theoretical perspectives to taxonomy.



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