Thursday, January 14, 2010

Thinking About Mechanisms (Part 3): Hierarchies, Schemata and Intelligibility

This post is part of my series on the work of the philosopher Carl Craver. The series focuses on the nature of neuroscientific explanations. For an index, see here.

I am currently looking at an article entitled "Thinking About Mechanisms", which Craver published with two other philosophers in 2000. The article introduces us to the contours of Craver's work.

Part One outlined Craver et al's formal analysis of mechanisms. Part Two cashed-out the value of this formal analysis by taking a detailed look at the depolarisation mechanism in chemical neurotransmission.

This final part will consider three philosophical issues: (1) the hierarchical nature of mechanistic explanations; (2) the importance of mechanism schemata in scientific discovery; and (3) the intelligibility of mechanistic explanations.

Before we begin, it is worth recalling the formal definition of a mechanism that is being appealed to:
Mechanisms are entities and activities organised such that they are productive of regular changes from start, or set-up, to finish, or termination.

(1) Hierarchies in Mechanistic Explanations
Mechanisms occur in nested hierarchies. This can be seen in the depolarisation example given in Part Two. The depolarisation mechanism is part of the mechanism for chemical neurotransmission, which is in turn a part of the mechanism for neuron-to-neuron signaling, and neuron-to-neuron signaling is part of virtually every cognitive mechanism.

This implies that mechanistic explanations are not necessarily reductive. Indeed, Craver et al argue that they work by linking together multiple levels of entities and activities. In this sense, mechanistic explanations are constructive, not reductive.

That said, the hierarchies do bottom-out at some point, namely: the lowest level entities and activities that are dealt with by a particular field of inquiry.

In molecular biology, the lowest level entities are the macromolecules, molecules and ions that are involved in biological processes. Molecular biology rarely peers below the atomic layer, but it is not an impassable barrier. As science progresses, the relevance of lower level entities may become more apparent.

As for the lowest level activities, the authors identify four that are of interest to the molecular biologist:
  1. Geometrico-mechanical activities: these are familiar from classical mechanics. They include activities such as fitting, turning, colliding, bending, pushing and so on.
  2. Electro-chemical activities: these are the attractions, repulsions and bondings that constitute the field of biochemistry.
  3. Energetic activities: these are activities that involve thermodynamic processes. For example, diffusion of molecules along a concentration gradient.
  4. Electro-magnetic activities: these are only occasionally used in molecular biology, but they are crucial to understanding the conduction of electrical impulses along nerve cells.

The authors argue that the history of science is largely the history of new mechanistic explanations at different levels in a hierarchy. Which brings us to the next point.

2. Mechanism Schemata and their Uses
A mechanism schemata is an abstract description of a type of mechanism. It usually pinpoints the entities and activities that are involved in the mechanism. An example (used by the authors) is Francis Crick's description of the central dogma of molecular biology. The authors attribute this to Jim Watson, but I'm sure it was Crick who originally formulated it. Anyway, it is illustrated below.

Crick's central dogma schema shows the entities and activities involved in protein synthesis (proteins being the essential building-blocks of biological cells). The schema is abstract because it does not contain a lot of detail; but it still has great explanatory scope because it explains a process shared by virtually all biological organisms (viruses are an exception).

Mechanism schemata of this nature are essential to scientific discovery for two reasons:
  • They can help to develop predictions. For example, the central dogma schema, with some detail about the DNA code, predict the order of amino acids in a protein.
  • They provide blueprints for designing research protocols. The experimenter can try to intervene in some part of the mechanism and observe the changes this results in.
The authors provide a lengthy illustration of these two virtues of mechanistic schemata by looking at the history of molecular biology. I am going to skip this. There will be plenty of time for detailed examples in later posts.

3. Intelligibility
The final point about mechanistic explanations is that they render the universe intelligible. They do so by setting out an elucidative relation between set-up conditions and intermediate activities (explanans) and termination conditions (explanandum). It is important to note that intelligibility has nothing to do with truthfulness: an intelligible explanation may be false. Intelligibility is all about cognitive attractiveness.

At the present moment in the history of molecular biology, the bottom-out activities and entities listed above occupy a privileged explanatory position: they have been used in numerous situations and subjected to numerous experimental manipulations. This could change.

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