Mechanisms and Explanation

Phyllis McKay and Jon Williamson

University of Kent

What is a mechanism?Bunge (2004, p186)

• Elementary physics: Electricity flows because the electric charges – electrons – in a wire are dragged by the impressed electric field – voltage.

• Chemistry: Synthesis of molecules from atoms most commonly happens by electron transfer (electrovalent bond) and electron sharing (covalent bond).

• Biology: Natural selection, the most important mechanism in evolutionary theory.

• Psychology: Extinction of aversive memories now thought to happen because cannabioids wreck the neuronal processes in the amygdale that store them.

• Economics: A market is a concrete system whose central mechanism is the exchange of goods and services.

The Mechanisms Literature

• Machamer, Darden and Craver: ‘Mechanisms are entities and activities organized such that they are productive of regular changes from start or set-up to finish or termination conditions.’ (MDC 2000 p3.)

• Glennan: ‘A mechanism for a behavior is a complex system that produces that behavior by the interaction of a number of parts, where the interactions between parts can be characterized by direct, invariant, change-relating generalizations.’ (Glennan 2002 pS344.)

• Bechtel and Abrahamsen: ‘A mechanism is a structure performing a function in virtue of its component parts, component operations, and their organization. The orchestrated functioning of the mechanism is responsible for one or more phenomena.’ (Bechtel and Abrahamsen 2005 p423.)

Two senses of Explanation

1. Epistemic: a human practice, aimed at increasing understanding of the world. Often involves the passing of information between people. Highly sensitive to the cognitive abilities and background knowledge of those improving, receiving and giving the information. Description of mechanism does the explaining.

2. Physical: mechanisms produce or are responsible for their phenomena. Independent of whether human beings understand or even observe them. Mechanism itself does the explaining.

The distinction is made in the literature (explicitly in Craver 2007). Not always carefully maintained.

To explain, mechanisms must be:

1. Real• Entities that exist in the world that produce

phenomena for the most part independently of how we describe them or of whether we even observe them.

• Note that independence comes in degrees: consider social mechanisms.

2. Local• Claim supported by examining decompositional

mechanistic explanation – to be explored.

Locality

• Broadly nearby in space and time– Protein synthesis happens in the cell where proteins are produced.– Natural selection happens in the population which undergoes

evolutionary change.

• Indexed to the phenomenon produced– Emission of gravitational waves versus social mechanisms.

• Note decompositional not etiological explanation• NOT

– a claim about intrinsicality imported from metaphysics– a locality claim imported from physics.

Two kinds of locality

• Local• Constitutive locality : what a mechanism is

depends only on things local to the mechanism.

• Productive locality: what produces the phenomena of interest is only the mechanism.

How explanation constrains the metaphysics of mechanisms

Two main approaches to the metaphysics of interactions

• Passive– Use some counterfactual notion to characterize the

interactions

– Use best-system laws or modal realist grounding for counterfactuals

• Active– MDC activities approach (bonding, breaking, binding).

– Cartwright’s capacities approach.

– Gillett’s powers view.

Passive views non-local

• Best-system laws– Interactions depend on a system of laws spanning the

whole universe.

– Interactions also depend in some sense on simplicity and strength of those laws.

• Modal realism– Interactions depend not only on this world, but also on

other possible worlds.

• Characterizing versus analyzing response

An active view like Cartwright’s can be local (and real)

• General causal claims like ‘aspirins relieve headaches’ really ascriptions of capacities.

• Capacities have properties and they are real, not abstract.• Ceteris paribus ‘laws’ arise out of reasonably regular

reaction of entities with similar capacities, not vice versa.• Capacities are properties of the entities in the mechanism,

quite independent of the rest of the world.• Mechanisms on such a metaphysics are robustly

constitutively and productively local.

Objection

Objection: Mechanisms are still local

• Status of mechanisms as mechanisms, and what they produce, depends only on their natural properties.

• These natural properties are local, and do their producing locally, despite the fact that they only interact with anything in virtue of laws widely spread in spacetime, or counterfactual notions grounded in other possible worlds.

What is an account of mechanisms really trying to do?

• If Woodward, Psillos and Glennan are really trying to make claims about deeper metaphysical nature of mechanisms, then their view implies the collapse of the distinction between mechanisms-based and law-based explanation.

Quotes on status of claim

• Glennan: ‘ “Interaction” is a causal notion that must be understood in terms of the truth of certain counterfactuals. The stipulation that these interactions can be characterized by invariant, change-relating generalizations is meant to capture the relevant counterfactual truth claims.’ (Glennan 2002 S344, emphasis added.)

• Psillos: ‘mechanisms need counterfactuals; but counterfactuals do not need mechanisms. In other words, mechanistic causation requires counterfactual dependence but not conversely. It is in this sense, that the counterfactual approach is more basic than the mechanistic.’ (Psillos p315.)

Quotes on collapse

• Glennan: ‘Laws such as these, which I call fundamental laws, represent brute nomological facts of our universe.’ (Glennan S348.)

• Psillos: ‘One plausible thought is that the fundamental laws govern the interactions of the parts of the mechanism, which realizes the non-fundamental law. If this is so (as I think it is), then it would be odd to say that the mechanism that explains, say, Ohm’s law is ultimately determined (supervenience is a kind of determination) by the fundamental laws that govern the interaction of fundamental particles but that these fundamental laws are not (part of) the truth-makers of Ohm's law. Once identified, the mechanism might well have explanatory and epistemic autonomy. But, if supervenience holds, the mechanism does not have metaphysical autonomy.’ (Psillos p310.)

The need for distinctively mechanistic explanation

• Reason 1: Fits the practice of the special sciences.• Reason 2: No laws – exceptionless non-accidental

generalizations in the special sciences. Reductive faith that there must be such laws sometime been empirically seriously undermined and should be abandoned.

• Reason 3: Hard to see how laws could explain in the way that mechanisms explain.– Brute nomological facts themselves call for

explanation. Do ascriptions of capacities?– Unification and epistemic vs physical explanation.

Heartfelt thanks to:– The Leverhulme Trust, for funding this project.– Jon, my co-author.– Many colleagues and students for enthusiastic

discussion.