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Mechanistic Explanation in Cognitive Science and Neuroscience

Edouard Machery University of Pittsburgh

Three Issues

(1) What is a mechanistic explanation?

(2) Are explanations in neuroscience mechanistic?

(3) How are explanations in neuroscience and in psychology related?

Plan

1. Mechanistic explanation

2. Explanation in neuroscience

3. The relation of psychology and neuroscience

Plan




A Refresher on Explanation

http://plato.stanford.edu/entries/scientific-explanation/

Hempel’s Deductive-Nomological (DN) Model of Explanation

Explanantia: Sentence (in a language L) of a Law(s) of Nature Sentence describing the initial conditions .

Explanandum: Sentence describing the event or generalization to be explained

But Explanation is Asymmetric

h

l

θ

Relevance and Explanation

All males who take birth control pills regularly fail to get pregnant !

John Jones is a male who has been taking birth control pills regularly !

John Jones fails to get pregnant !

(Salmon, 1981)

Underlying Issue

What seems amiss is that we want to explain an event by citing its causes:

- causes are asymmetric

- causes are relevant

Lack of Laws

It is unclear what laws are in biology, psychology, sociology, etc.

Upshot

Another model of explanation is needed, which acknowledges the explanatory role of causes and

does not require laws.

Mechanistic Explanation: The MDC Model

Phil Sci 2000

Mechanism

Mechanism

Mechanism

A mechanism is a physical object that does something (for artifacts, its function) by virtue of the organization of its parts and of their activities

(what they do).

Mechanisms in Science

Mechanistic Explanation

To explain a phenomenon is to describe the mechanism producing it

Mechanistic Explanation

That is, (1) to identify the parts of a mechanism and (2) their activities, which produce the

phenomenon to be explained.

Protein Synthesis

Mechanistic Explanations and then Failure of the DN Model

The mechanistic model of explanation acknowledges the explanatory role of causes and does not require laws, from which a phenomenon

can be deduced.

No Laws At All?

Generalizations describing the behavior of parts are however needed: “Laws in situ” (Cummins) or “mechanistically fragile generalizations” (Craver).

Constitutive Explanation

Mechanistic explanations are constitutive (in contrast to merely etiological): An explanation of

a phenomenon P involves determining what a mechanism is.

Decomposition and Recomposition

Discovering explanations depends on two complementary discovery heuristics: decomposition and recomposition.

Multi-Level Explanation

Mechanistic explanations are hierarchical and multi-level in that the activities of the parts of

mechanisms need to be explained as well.

Plan




The Issue

Are explanations in neuroscience mechanistic?

Explanation in Neuroscience

Explanations in neuroscience are mechanistic.

Craver, chap. 4

Case Study: The Action Potential





The discovery of the explanation of the action potential consists in understanding the

mechanism responsible for it: replacing “filler terms” with mechanism parts and their

interactions, etc.

Contrast with Cummins’s Functional Explanation

To explain a capacity φ of a system (characterized extensionally as a I-O function) is to identify a set of subcapacities that together

constitute φ.

A Narrow Perspective on Explanation in Cognitive Science and

Neuroscience

It is dubious that all explanations in neuroscience are mechanistic. In particular, computational

explanations are not mechanistic.

Example: Nosofsky’s (1986) Generalized Context Model

Nosofsky 1986


Nosofsky 1986


Nosofsky 1986

Plan




The Issue

Are psychological explanations (particularly, the kind of explanation provided in cognitive science) autonomous from neuroscientific explanations?

The Claim

Psychological explanations are not autonomous because they are mechanisms sketches

(incomplete, to-be-filled-in) of neuroscientific mechanistic explanations.

The Target

The Argument1. Psychological explanations are functional explanations.

2. Functional explanations are sketches of mechanistic explanations.

3. Mechanistic explanations of cognitive competences are neuroscientific explanations.

4. If type-A explanations are sketches of type-B explanations, type-A explanations improve (get more explanatory) as their explanantia are connected to type-B explanantia

5. type-A explanations are autonomous from type-B explanations iff type-A explanations do not improve (get more explanatory) as their explanantia are connected to type-B explanantia.

6. Psychological explanations are not autonomous.





5. Type-A explanations are autonomous from type-B explanations iff type-A explanations do not improve (get more explanatory) as their explanantia are connected to type-B explanantia.


Functional Explanation

Explaining a capacity by “functional analysis” consists in identifying subcapacities and their organization, which are constitutive of the possession of the capacity to be explained.

Example

Functional Explanation in Psychology

Explaining a psychological capacity by “functional analysis” consists in identifying psychological

subcapacities and their organization, which are constitutive of the possession of the capacity to

be explained.

Example

Coltheart et al. 2001

Optimality Explanation

Sanborn et al. 2001

Optimality Explanation

Sanborn et al. 2001



















Autonomy

Different types of autonomy:

(1) ontological autonomy

(2) evidential autonomy

(3) explanatory autonomy

Evidential Heteronomy

(1) Findings at a lower level can confirm or undermine high-level theories.

(2) Findings at a lower level are necessary to confirm or undermine high-level theories.

Explanatory Heteronomy

(Potential) explanations couched in higher-level terms improve to the extent that their explanantia are connected with (shown to be identical to or

constituted by) the explanantia of lower-level explanations.

Evidential and Explanatory Heteronomy

Even if psychology is evidentially heteronomous in either sense (1) or (2), it does not entail that it is

explanatory heterogeneous.







P&C’s Strategy

Examine three forms/presentations* of psychological explanations:

- task analysis

- functional analysis

- boxological models

P&C’s Argument for Premise 2

1. A system S has a particular capacity because of the capacities of its parts and their organization.

2. Hence, the subcapacities that constitute S’s capacity are capacities of the parts of a system.

3. Hence, a functional explanation is a mechanism sketch.

P&C’s Argument for Premise 2

Problem

1 does not entail 2:

While the HL properties of a system depend on the properties of LL parts, they need not be

properties of these parts.

Problem

Problem

Properties of societies and properties of individuals

Upshot

Functional analyses can, but need not be, mechanism sketches.

Upshot

But the argument against the autonomy of psychology requires that functional analyses

essentially are mechanism sketches.

Upshot

So, this argument fails.

Conclusion

Still little reason to deny the explanatory autonomy of psychology.

Claims to the contrary confuse dependence or evidential relations with explanatory ones.

Two Distinctions

1. How-possibly vs. how-actually mechanistic explanations

2. Mechanism sketches vs. mechanism schemata.

How-Possibly vs. How-Actually Mechanistic Explanations

How possibly

- describe a mechanism that might be responsible for the explanandum

- heuristically useful

- does not explain adequately


1. ConfusionsSchemata Sketches

how possibly

Complete description of

a possible mechanism

Incomplete description of

a possible mechanism

how actually

Complete description of

the mechanism

Incomplete description of

the mechanism

How actually

- describe a component that is responsible for the explanandum

- explains adequately


Mechanisms Sketches vs. Mechanism Schemata

Mechanisms Sketches vs. Mechanism Schemata

Sketches

Some components or capacities have not been specified.

1. Confusions

Craver mischaracterizes the distinction between how-possibly and how-actually mechanism

descriptions.

1. Confusions

Two distinctions:

- how possibly and how actually mechanism description.

- successful vs. unsuccessful explanation

1. Confusions

How-possibly vs. how-actually mechanism descriptions

Explaining how a phenomenon might be produced vs. is produced

1. ConfusionsSuccessful Unsuccessful

how possibly

epistemically possible and sufficient for

the phenomenon

epistemically impossible or insufficient for

the phenomenon

how actually

true description of

the mechanism

false or incomplete

description of the mechanism

1. Confusions

Craver tends to identify how-possibly mechanism descriptions with mechanism sketches and how-actually mechanism descriptions with mechanism

schemata.

Response

Rebuttal

Functional analyses that are not mechanism sketches are neither actual nor merely possible description of the causal structure of system, if a description of a causal structure identifies parts

of a system and their causal interactions.

Rebuttal

Unclear why P&C take the first horn of the dilemma to be true, but a possible explanation is

that they take functional analysis to be underdetermined without evidence about the

mechanism bringing about the capacity.

Rebuttal

But even if this underdetermination is true, it is irrelevant to the issue at hand. We may use

information about parts of a system to identify the one true functional analysis, but this analysis need not be a mechanism sketch in that it need

not describe properties of the parts of the mechanismm.

P&C’s Confusion

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