1111111 Mechanisms and Modularity: An Interventionist Approach [email protected].

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Mechanisms and Modularity: An

Interventionist Approach

Mechanisms and Modularity: An

Interventionist [email protected]@mq.edu.au

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1. Introduction1. Introduction

The concept of a mechanism has been often been invoked in theories of causation as providing the key to ‘the secret connexion’ between cause and effect. Cf Mackie on persistence of a quality, Salmon on causal processes and interaction, and Dowe on conserved quantities.

More recently, the concept has been invoked by philosophers of science to express the view that much explanation in the sciences, especially in the biomedical and social sciences, is mechanistic explanation.

The concept of a mechanism has been often been invoked in theories of causation as providing the key to ‘the secret connexion’ between cause and effect. Cf Mackie on persistence of a quality, Salmon on causal processes and interaction, and Dowe on conserved quantities.

More recently, the concept has been invoked by philosophers of science to express the view that much explanation in the sciences, especially in the biomedical and social sciences, is mechanistic explanation.

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The view is that an important form of explanation in these sciences consists in describing the mechanism that generates some phenomenon.

Roughly, a mechanism is a set of entities and activities organized such that they exhibit the phenomenon to be explained.

These philosophers claim that recognizing this distinctive form of explanation enables us to understand many aspects of scientific practice.

The important works in this area include: Wimsatt 1972, Bechtel and Richardson 1993, Glennan 1996, Machamer,Darden and Craver 2000, Craver 2007, Bechtel 2008.

The view is that an important form of explanation in these sciences consists in describing the mechanism that generates some phenomenon.

Roughly, a mechanism is a set of entities and activities organized such that they exhibit the phenomenon to be explained.

These philosophers claim that recognizing this distinctive form of explanation enables us to understand many aspects of scientific practice.

The important works in this area include: Wimsatt 1972, Bechtel and Richardson 1993, Glennan 1996, Machamer,Darden and Craver 2000, Craver 2007, Bechtel 2008.

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Mechanistic explanation differs from covering law explanation in that:

The aim of mechanistic explanation is to reveal the underlying mechanism for some phenomenon rather than to bring it under a covering law.

Mechanistic explanations describe how some phenomenon exhibited by a system is constituted by its component parts engaging in distinctive activities and causal interactions.

The activities and interactions of the constituents conform to generalizations, but these generalizations are narrow in scope, have exceptions, and are historically contingent.

Mechanistic explanation differs from covering law explanation in that:

The aim of mechanistic explanation is to reveal the underlying mechanism for some phenomenon rather than to bring it under a covering law.

Mechanistic explanations describe how some phenomenon exhibited by a system is constituted by its component parts engaging in distinctive activities and causal interactions.

The activities and interactions of the constituents conform to generalizations, but these generalizations are narrow in scope, have exceptions, and are historically contingent.

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1. Introduction1. Introduction In this paper I plan to critically scrutinize this conception of

mechanistic explanation, concentrating on the work of Carl Craver (2007).

Even though his account pays more attention than is usual to the causal structure of mechanistic explanation, it omits important details.

The crucial causal structure of mechanistic explanation can be captured by an interventionist account of causation. It highlights a modularity requirement to the effect that the components of a mechanism should be separately modifiable.

I shall illustrate this requirement by considering a typical information-processing model from cognitive psychology where this requirement plays a vital role.

Finally, I shall consider some objections to the modularity requirement and show how Pearl’s interventionist account is better placed than Woodward’s version to respond to them.

In this paper I plan to critically scrutinize this conception of mechanistic explanation, concentrating on the work of Carl Craver (2007).

Even though his account pays more attention than is usual to the causal structure of mechanistic explanation, it omits important details.

The crucial causal structure of mechanistic explanation can be captured by an interventionist account of causation. It highlights a modularity requirement to the effect that the components of a mechanism should be separately modifiable.

I shall illustrate this requirement by considering a typical information-processing model from cognitive psychology where this requirement plays a vital role.

Finally, I shall consider some objections to the modularity requirement and show how Pearl’s interventionist account is better placed than Woodward’s version to respond to them.

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2. Craver’s Account2. Craver’s Account

On Craver’s account, a mechanistic explanation describes a mechanism:A mechanism for the phenomenon of a system S’s -ing consists in a set of parts of S, X1,…,Xn, and a set of distinctive activities 1,…, n that are organized so as to exhibit S’s -ing.

Craver describes activities as i-ings and as arrows.

On Craver’s account, a mechanistic explanation describes a mechanism:A mechanism for the phenomenon of a system S’s -ing consists in a set of parts of S, X1,…,Xn, and a set of distinctive activities 1,…, n that are organized so as to exhibit S’s -ing.

Craver describes activities as i-ings and as arrows.

S -ing

X4 4-ingX1 1-ing

X2 2-ing

X3 3-ing

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2. Craver’s Account2. Craver’s Account

The explanandum phenomenon: S’s ing. This is characterized by input and output conditions.

Component entities: X1,…, Xn. These are constituents or parts of the system S.

Activities: 1,…, n. Craver says these are things done by the Xis. “Activities are the causal components in mechanisms.”

Organization: the entities and their activities are causally, temporally, and spatially organized in such a way as to exhibit the explanandum phenomenon.

The explanandum phenomenon: S’s ing. This is characterized by input and output conditions.

Component entities: X1,…, Xn. These are constituents or parts of the system S.

Activities: 1,…, n. Craver says these are things done by the Xis. “Activities are the causal components in mechanisms.”

Organization: the entities and their activities are causally, temporally, and spatially organized in such a way as to exhibit the explanandum phenomenon.

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2. Craver’s Account 2. Craver’s Account

Explanandum phenomenon: a neuron’s release of neurotransmitter when depolarized.

Component entities: axon terminals, synaptic clefts, calcium ion channels,vesicles, neural membranes

Activities: ion channel’s opening, vesicle’s docking, opening, diffusing,

Causal Interactions: arrival of action potential opens calcium channels in presynaptic membrane, the influx of calcium increases calcium concentration in interior, high calcium concentration causes calcium-sensitive proteins attached to vesicles to change shape, their changed shape causes the membranes of ‘docked’ vesicles to fuse with membrane this fusion causes vesicles to release neurostransmiitters into synaptic cleft.

Explanandum phenomenon: a neuron’s release of neurotransmitter when depolarized.

Component entities: axon terminals, synaptic clefts, calcium ion channels,vesicles, neural membranes

Activities: ion channel’s opening, vesicle’s docking, opening, diffusing,

Causal Interactions: arrival of action potential opens calcium channels in presynaptic membrane, the influx of calcium increases calcium concentration in interior, high calcium concentration causes calcium-sensitive proteins attached to vesicles to change shape, their changed shape causes the membranes of ‘docked’ vesicles to fuse with membrane this fusion causes vesicles to release neurostransmiitters into synaptic cleft. 88

Axon terminal

Synapticcleft

Dendritic spine

Neurotransmitters

NeurotransmitterRe-uptake pump

Neurotransmitterreceptors

Synaptic vesicle

Voltage-gatedC2+ channels

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3. The Causal Character of Mechanistic Explanation

3. The Causal Character of Mechanistic Explanation

Craver emphasizes that account of mechanistic explanation should not only describe scientific practice but also provide appropriate norms for evaluating explanations.

However, his own account doesn’t provide all the appropriate norms because it doesn’t capture all the relevant causal features of mechanistic explanation.

His account doesn’t completely explain the point or function of mechanistic explanation; and so can’t explain why it is so important and prevalent in science.

Craver emphasizes that account of mechanistic explanation should not only describe scientific practice but also provide appropriate norms for evaluating explanations.

However, his own account doesn’t provide all the appropriate norms because it doesn’t capture all the relevant causal features of mechanistic explanation.

His account doesn’t completely explain the point or function of mechanistic explanation; and so can’t explain why it is so important and prevalent in science.

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3. The Causal Character3. The Causal Character3.1 What is the explanandum phenomenon?Generally, an explanandum phenomenon will consist in a range

of capacities of a system, as specified by an array of input-output conditions.

In example of release of neurotransmitters these might include:precipitating conditions: the different ways action potentials can be formed inhibiting conditions: the different kinds of preventers of the release of neurotransmitters (eg raising concentration of magnesium ions blocks influx of calcium ions.)

modulating conditions: the different kinds of conditions affecting the quanta of neurotransmitters released.

I agree that the target explanandum phenomenon consists in the capacities of some system, as specified in terms of a battery of causally sensitive counterfactuals.

3.1 What is the explanandum phenomenon?Generally, an explanandum phenomenon will consist in a range

of capacities of a system, as specified by an array of input-output conditions.

In example of release of neurotransmitters these might include:precipitating conditions: the different ways action potentials can be formed inhibiting conditions: the different kinds of preventers of the release of neurotransmitters (eg raising concentration of magnesium ions blocks influx of calcium ions.)

modulating conditions: the different kinds of conditions affecting the quanta of neurotransmitters released.

I agree that the target explanandum phenomenon consists in the capacities of some system, as specified in terms of a battery of causally sensitive counterfactuals.

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3. The Causal Character3. The Causal Character3.2 What are the components’ activities?Craver says that “ Activities are the causal

components in mechanisms. I use the term ‘activity’ as a filler-term for productive behaviors, causal interactions, omissions, preventions and so on.”

Examples of activity statements: The arrival of an action potential at a cell’s axon

terminal opens calcium ion channels. High calcium concentration in the interior activates

a set of calcium-sensitive proteins attached to the vesicles.

The fused vesicles release their neurotransmitter contents into the synaptic cleft.

Craver says it’s essential that these statements describe causal relations, and not mere correlations or temporal sequences.

3.2 What are the components’ activities?Craver says that “ Activities are the causal

components in mechanisms. I use the term ‘activity’ as a filler-term for productive behaviors, causal interactions, omissions, preventions and so on.”

Examples of activity statements: The arrival of an action potential at a cell’s axon

terminal opens calcium ion channels. High calcium concentration in the interior activates

a set of calcium-sensitive proteins attached to the vesicles.

The fused vesicles release their neurotransmitter contents into the synaptic cleft.

Craver says it’s essential that these statements describe causal relations, and not mere correlations or temporal sequences.

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3. The Causal Character3. The Causal CharacterFollowing Woodward, Craver argues that these causal relations are

distinguished from correlations in that they are invariant under interventions. Causal relationships are essentially exploitable for the purposes of manipulation and control.

According to Woodward, these causal relationships can be explicated in terms of interventionist counterfactuals = counterfactuals whose antecedents are realized by an intervention.

Roughly speaking, X is causally relevant to Y if and only if there is some possible intervention such that if it were to change X it would also change Y.

A rise in intracellular calcium ions is causally relevant to the release of neurotransmitters because if an intervention were to raise the level of intracellular calcium ions, neurotransmitters would be released.

A rise in intracellular sodium ions is merely correlated with the release of neurotransmitters because if an intervention were to raise the level of intracellular sodium ions, neurotransmitters would not be released.

Following Woodward, Craver argues that these causal relations are distinguished from correlations in that they are invariant under interventions. Causal relationships are essentially exploitable for the purposes of manipulation and control.

According to Woodward, these causal relationships can be explicated in terms of interventionist counterfactuals = counterfactuals whose antecedents are realized by an intervention.

Roughly speaking, X is causally relevant to Y if and only if there is some possible intervention such that if it were to change X it would also change Y.

A rise in intracellular calcium ions is causally relevant to the release of neurotransmitters because if an intervention were to raise the level of intracellular calcium ions, neurotransmitters would be released.

A rise in intracellular sodium ions is merely correlated with the release of neurotransmitters because if an intervention were to raise the level of intracellular sodium ions, neurotransmitters would not be released.

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3. The Causal Character3. The Causal Character

An intervention I on X with respect to Y is a change in the value of X that changes Y, if at all, only via the change in X. More specifically:(I1) I does not change Y directly;(I2) I does not change the value of some causal intermediate S between X and Y except by changing some value of X;(I3) I is not correlated with some variable C that is a cause of Y;

(I4) I acts as a “switch” that controls the value of X

irrespective of X’s other causes, U.

An intervention I on X with respect to Y is a change in the value of X that changes Y, if at all, only via the change in X. More specifically:(I1) I does not change Y directly;(I2) I does not change the value of some causal intermediate S between X and Y except by changing some value of X;(I3) I is not correlated with some variable C that is a cause of Y;

(I4) I acts as a “switch” that controls the value of X

irrespective of X’s other causes, U. U X

I

S Y

C

I4

I2

I1

I3

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3. The Causal Character3. The Causal Character3.3 What is the organization of component parts?Craver emphasizes the spatial organization (sizes, shapes, locations,

orientations, directions, connections of components) and temporal (order, rate, duration of activities) organization of a mechanism.

But he says very little about the causal organization of the components. But this is crucial because the components must causally interact in such a way that when the inputs of the phenomenon are produced the outputs are generated.

However, he does address a related question: Which parts of a system count as components of the mechanism? He emphasizes the role of intervention experiments in answering this question and distinguishes between two kinds of such experiment.

3.3 What is the organization of component parts?Craver emphasizes the spatial organization (sizes, shapes, locations,

orientations, directions, connections of components) and temporal (order, rate, duration of activities) organization of a mechanism.

But he says very little about the causal organization of the components. But this is crucial because the components must causally interact in such a way that when the inputs of the phenomenon are produced the outputs are generated.

However, he does address a related question: Which parts of a system count as components of the mechanism? He emphasizes the role of intervention experiments in answering this question and distinguishes between two kinds of such experiment.

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3. The Causal Character3. The Causal CharacterBottom-up Experiment Top-down Experiment

Craver’s criterion of constitutive relevance: a part is a component in a mechanism if one can change the behaviour of the mechanism as a whole by intervening to change the component and one can change the behaviour of the component by intervening to change the mechanism as a whole.

Bottom-up Experiment Top-down Experiment

Craver’s criterion of constitutive relevance: a part is a component in a mechanism if one can change the behaviour of the mechanism as a whole by intervening to change the component and one can change the behaviour of the component by intervening to change the mechanism as a whole.

Intervention

detection

detection

Intervention

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3. The Causal Character3. The Causal Character

We can infer Craver’s views about the causal organization of a mechanism from his account of constitutive relevance.

First, the point of the different kinds of intervention experiments is to reveal the internal causal structure of the mechanism: to reveal which activities are causally between the inputs and outputs of the explanandum phenomenon and to reveal their internal causal order.

Second, the success of intervention experiments in achieving this goal presupposes that the components of the mechanism are modular in the sense that it is possible to intervene to change the behaviour of one component without interfering with the behaviour of others.

We can infer Craver’s views about the causal organization of a mechanism from his account of constitutive relevance.

First, the point of the different kinds of intervention experiments is to reveal the internal causal structure of the mechanism: to reveal which activities are causally between the inputs and outputs of the explanandum phenomenon and to reveal their internal causal order.

Second, the success of intervention experiments in achieving this goal presupposes that the components of the mechanism are modular in the sense that it is possible to intervene to change the behaviour of one component without interfering with the behaviour of others.

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4. A Structural Equations Account of Mechanism

4. A Structural Equations Account of Mechanism

It’s possible to provide a more systematic account of the causal structure of a mechanism that builds on Craver’s beginnings. I use the tools of causal modeling and especially of structural equations modeling.

A causal model consists of an ordered pair <V, E>, where V is a set of variables and E is a set of structural equations.

Consider the way in which Craver’s example of neurotransmitters might be represented in this framework:

AP = 1 if action potential arrives at axon; 0 otherwise.CC = 1 if calcium channels open in cell membrane; 0 otherwise.P = 1 if calcium-sensitive proteins attached to vesicles change shape; 0 otherwise.F = 1 if vesicles fuse with presynaptic membrane.NR=1 if neurotransmitters in vesicles released into synaptic cleft.

It’s possible to provide a more systematic account of the causal structure of a mechanism that builds on Craver’s beginnings. I use the tools of causal modeling and especially of structural equations modeling.

A causal model consists of an ordered pair <V, E>, where V is a set of variables and E is a set of structural equations.

Consider the way in which Craver’s example of neurotransmitters might be represented in this framework:

AP = 1 if action potential arrives at axon; 0 otherwise.CC = 1 if calcium channels open in cell membrane; 0 otherwise.P = 1 if calcium-sensitive proteins attached to vesicles change shape; 0 otherwise.F = 1 if vesicles fuse with presynaptic membrane.NR=1 if neurotransmitters in vesicles released into synaptic cleft.

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4. Structural Equations4. Structural EquationsIn this simple model, there is one exogenous variable AP =1. For

every endogenous variable, there is structural equation of the form Y = f(X1,…,Xn) where X1,…,Xn are all the direct causes of Y.

CC = f(AP)P = g(CC)F = h(P)NR = i(F)

Pearl says that if these equations are to be interpreted causally they must be invariant under interventions on variables on the right-hand side (but not on the left-hand side). If correct, the equation describes what Pearl calls a mechanism. But I shall say a correct equation describes a causal capacity of a system.

Structural equations imply interventionist counterfactuals. To evaluate the interventionist counterfactual ‘If F=0 were the case, it would be NR=0’, one replaces the structural equation for F with F =0 and then solves the remaining equations.

An intervention that realizes the counterfactual antecedent overrides the normal causal factors that would influence this variable.

In this simple model, there is one exogenous variable AP =1. For every endogenous variable, there is structural equation of the form Y = f(X1,…,Xn) where X1,…,Xn are all the direct causes of Y.

CC = f(AP)P = g(CC)F = h(P)NR = i(F)

Pearl says that if these equations are to be interpreted causally they must be invariant under interventions on variables on the right-hand side (but not on the left-hand side). If correct, the equation describes what Pearl calls a mechanism. But I shall say a correct equation describes a causal capacity of a system.

Structural equations imply interventionist counterfactuals. To evaluate the interventionist counterfactual ‘If F=0 were the case, it would be NR=0’, one replaces the structural equation for F with F =0 and then solves the remaining equations.

An intervention that realizes the counterfactual antecedent overrides the normal causal factors that would influence this variable.

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4. Structural Equations4. Structural EquationsThe relationships among variables can be captured in a

directed graph. An arrow is drawn from one variable to another when the first appears on the RHS of the equation for the second.

A series of arrows from tip to tail constitutes a path through the graph. A graph is acyclic just in case no path loops back on itself.

Here is the directed graph for the neurotransmitter example:

AP CC P F NR

We can also represent the result of an intervention graphically(arrow-breaking convention):

I

AP CC P F NR

The relationships among variables can be captured in a directed graph. An arrow is drawn from one variable to another when the first appears on the RHS of the equation for the second.

A series of arrows from tip to tail constitutes a path through the graph. A graph is acyclic just in case no path loops back on itself.

Here is the directed graph for the neurotransmitter example:

AP CC P F NR

We can also represent the result of an intervention graphically(arrow-breaking convention):

I

AP CC P F NR

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4. Structural Equations4. Structural Equations

A set of structural equations is modular if and only if it is possible to replace any of the equations in the system by means of an intervention without disrupting any of the other equations.

It’s reasonable to think that if a system of equations correctly and fully represents the causal structure of some mechanism, then those equations should be modular.

Each equation should represent the operation of a distinct causal capacity.

A necessary condition for two causal capacities to be distinct is that it be possible to interfere in the operation of one without interfering with the operation of the other and vice versa.

A set of structural equations is modular if and only if it is possible to replace any of the equations in the system by means of an intervention without disrupting any of the other equations.

It’s reasonable to think that if a system of equations correctly and fully represents the causal structure of some mechanism, then those equations should be modular.

Each equation should represent the operation of a distinct causal capacity.

A necessary condition for two causal capacities to be distinct is that it be possible to interfere in the operation of one without interfering with the operation of the other and vice versa.

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4. Structural Equations4. Structural EquationsThe causal structure of mechanism which underlies a system’s

capacity O = f(11,…,In) is represented by a modular set of structural equations E1,…,Em each of which correctly describes a capacity of the system and all of which compose to yield the function O = f(11,…,In).

In material mode, the causal structure of a mechanism is given by a set of modular subcapacities whose sequential exercise has the input-output profile of the capacity to be explained.

Example: explanandum capacity of a system 0 = f(I):

This set of equations captures of the causal structure of the mechanism underlying the explanandum phenomenon just in case each of the structural equations correctly describes a subcapacity of the system and these equations compose to yield the explanandaum capacity: ie.

f(I) = O = j(g(I), h(I))

The causal structure of mechanism which underlies a system’s capacity O = f(11,…,In) is represented by a modular set of structural equations E1,…,Em each of which correctly describes a capacity of the system and all of which compose to yield the function O = f(11,…,In).

In material mode, the causal structure of a mechanism is given by a set of modular subcapacities whose sequential exercise has the input-output profile of the capacity to be explained.

Example: explanandum capacity of a system 0 = f(I):

This set of equations captures of the causal structure of the mechanism underlying the explanandum phenomenon just in case each of the structural equations correctly describes a subcapacity of the system and these equations compose to yield the explanandaum capacity: ie.

f(I) = O = j(g(I), h(I))

I O

X

Y

X= g(I)Y = h(I)0 = j(X,Y)

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5. Example from Psychology5. Example from PsychologyThe causal structure of a mechanism can be represented by a set of

structural equations. We can represent each structural equation by a module. Mechanistic explanation corresponds to opening up the module to reveal its inner workings.

Cf Cummins on functional analysis: a psychological capacity is explained by analyzing it into information-processing subcapacities whose programmed exercise produces the capacity in question.

Contra Craver, this is a form of mechanistic explanation. The mechanistic decomposition of a system’s capacity can proceed in terms of subcapacities of the whole system, and not just in terms of subcapacities of the system’s components.

This is not deny there is more to mechanistic explanation than the description of internal causal structure. Spatial and temporal organization are important.

The causal structure of a mechanism can be represented by a set of structural equations. We can represent each structural equation by a module. Mechanistic explanation corresponds to opening up the module to reveal its inner workings.

Cf Cummins on functional analysis: a psychological capacity is explained by analyzing it into information-processing subcapacities whose programmed exercise produces the capacity in question.

Contra Craver, this is a form of mechanistic explanation. The mechanistic decomposition of a system’s capacity can proceed in terms of subcapacities of the whole system, and not just in terms of subcapacities of the system’s components.

This is not deny there is more to mechanistic explanation than the description of internal causal structure. Spatial and temporal organization are important.

I O IO

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5. Example from Psychology5. Example from PsychologyThe present account of mechanisms puts the modularity requirement

front and centre. In this respect, the account captures an important feature of mechanistic explanations in cognitive psychology.

As an example, consider an information-processing model of reading, which is presented as a model of “mental machinery that enables reading” (Coltheart 2006):

The lexical module accepts input from all letters. It’s a dictionary look-up procedure that works only for real words.

The nonlexical module translates letters to sound by left-to-right application of letter-to-sound rules. This is required for reading aloud nonwords.

The present account of mechanisms puts the modularity requirement front and centre. In this respect, the account captures an important feature of mechanistic explanations in cognitive psychology.

As an example, consider an information-processing model of reading, which is presented as a model of “mental machinery that enables reading” (Coltheart 2006):

The lexical module accepts input from all letters. It’s a dictionary look-up procedure that works only for real words.

The nonlexical module translates letters to sound by left-to-right application of letter-to-sound rules. This is required for reading aloud nonwords.

Mental lexicon

Letter identification

Non-lexical

process

print

speech

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5. Example from Psychology5. Example from PsychologyThe meaning of modularity is much contested in cognitive

psychology. According to Fodor (1983), a module is typically autonomous, domain specific, innately specified, informationally encapsulated, mandatory, fast, and hardwired (neurally specific).

I think that the basic meaning of modularity is that of autonomy: a process is modular just when it can be modified independently of other processes. This usage is widespread in psychology.

“Two (sub)processes A and B of a complex process (mental or neural) are modules if and only if each can be changed independently of the other…To demonstrate separate modifiability of A and B, we must find experimental manipulations F and G that influence A and B selectively, i.e.such that A is influenced by F but is invariant with respect to G, whereas B is influenced by G but is invariant with respect to F.” (Sternberg 2010)

Other features of modularity presuppose this notion and then add extra empirical assumptions to it. Sternberg moves from autonomous (independently modifiable) to a domain specific (responsive to distinctive stimuli).

The meaning of modularity is much contested in cognitive psychology. According to Fodor (1983), a module is typically autonomous, domain specific, innately specified, informationally encapsulated, mandatory, fast, and hardwired (neurally specific).

I think that the basic meaning of modularity is that of autonomy: a process is modular just when it can be modified independently of other processes. This usage is widespread in psychology.

“Two (sub)processes A and B of a complex process (mental or neural) are modules if and only if each can be changed independently of the other…To demonstrate separate modifiability of A and B, we must find experimental manipulations F and G that influence A and B selectively, i.e.such that A is influenced by F but is invariant with respect to G, whereas B is influenced by G but is invariant with respect to F.” (Sternberg 2010)

Other features of modularity presuppose this notion and then add extra empirical assumptions to it. Sternberg moves from autonomous (independently modifiable) to a domain specific (responsive to distinctive stimuli).

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5. Example from Psychology5. Example from PsychologyThe lexical and nonlexical reading modules are domain

specific: the nonlexical module is influenced by number of letters and the lexical module is not, whereas (in the case where what is being read is a word), the lexical module is influenced by the frequency of occurrence of the word whereas the nonlexical module is not.

On this model we can predict that: Nonword reading RT will increase as a function of

number of letters. Word reading RT will be insensitive to the number of

letters. The difference between word and nonword RTs will

increase as a function of the number of letters so when one averages across number of letters words should be faster than nonwords.

The lexical and nonlexical reading modules are domain specific: the nonlexical module is influenced by number of letters and the lexical module is not, whereas (in the case where what is being read is a word), the lexical module is influenced by the frequency of occurrence of the word whereas the nonlexical module is not.

On this model we can predict that: Nonword reading RT will increase as a function of

number of letters. Word reading RT will be insensitive to the number of

letters. The difference between word and nonword RTs will

increase as a function of the number of letters so when one averages across number of letters words should be faster than nonwords.

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5. Example from Psychology5. Example from Psychology

The mental lexicon contains at least three kinds of information about words: their spellings, their pronunciations, and their meanings. Neuropsychological research indicates that the three forms of information are stored in three separate systems, as shown below.

Information about the three separate modules is gained by natural experiments.People with dementia don’t know meanings of words but have normal visual word

recognition and normal pronunciation.People with anomia aphasia can’t access the pronunciation of words but have

normal visual word recognition and normal knowledge of word meanings.People with surface dyslexia can see but can no longer recognize familiar printed

words even though they are still able to speak words and still able to appreciate their meanings.

The mental lexicon contains at least three kinds of information about words: their spellings, their pronunciations, and their meanings. Neuropsychological research indicates that the three forms of information are stored in three separate systems, as shown below.

Information about the three separate modules is gained by natural experiments.People with dementia don’t know meanings of words but have normal visual word

recognition and normal pronunciation.People with anomia aphasia can’t access the pronunciation of words but have

normal visual word recognition and normal knowledge of word meanings.People with surface dyslexia can see but can no longer recognize familiar printed

words even though they are still able to speak words and still able to appreciate their meanings.

Letteridentification

Orthographclexicon

Semantcsystem

Phonologicallexicon

Grapheme-phonemeCorrespondence rules

print

speech

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5. Example from Psychology5. Example from Psychology

Much mechanistic explanation in cognitive psychology observes the modularity requirement.

As such, mechanistic explanation has typical benefits, which explain its prevalence and importance in psychology:(1) It identifies modules that may display more invariant behaviour than the systems to which they belong. (2) It identifies modules that can be studied in isolation as well as in situ.(3) It identifies modules that may be reassembled in other systems. (4) It isolates modules that can be independently debugged or repaired.

If psychological mechanisms were not modular, then these benefits could not be realized.

Much mechanistic explanation in cognitive psychology observes the modularity requirement.

As such, mechanistic explanation has typical benefits, which explain its prevalence and importance in psychology:(1) It identifies modules that may display more invariant behaviour than the systems to which they belong. (2) It identifies modules that can be studied in isolation as well as in situ.(3) It identifies modules that may be reassembled in other systems. (4) It isolates modules that can be independently debugged or repaired.

If psychological mechanisms were not modular, then these benefits could not be realized.

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6. Criticisms of Modularity6. Criticisms of ModularityI regard the modularity assumption as a default assumption we make in

studying mechanisms in cognitive psychology.

As evidence for this, I have cited the common experimental methods that take modules to be domain specific and functionally dissociable.

These depend on the assumption that an intervention (natural or experimental) can disrupt one module or capacity without disrupting others.

More generally, the modularity assumption is part of the general strategy of mechanistic explanation which tries to understand the workings of a whole system in terms of the workings of individual components, taken in isolation.

.

I regard the modularity assumption as a default assumption we make in studying mechanisms in cognitive psychology.

As evidence for this, I have cited the common experimental methods that take modules to be domain specific and functionally dissociable.

These depend on the assumption that an intervention (natural or experimental) can disrupt one module or capacity without disrupting others.

More generally, the modularity assumption is part of the general strategy of mechanistic explanation which tries to understand the workings of a whole system in terms of the workings of individual components, taken in isolation.

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6. Criticisms of Modularity6. Criticisms of Modularity

Nancy Cartwright (2007) has proposed counterexamples involving machines to the principles. See Woodward (2008), Steel (2010), and Kuorikoski (2010) for responses to these

examples. But she also advances an interesting thought experiment:

“…[given modularity] we can see that it is impossible to build a bomb that cannot be defused. Nor can we make a deterministic device of this sort: the correct functioning of the mechanism requires that they operate in a vacuum; so we seal the whole device in a vacuum in such a way that we cannot penetrate the cover to affect one cause in the chain without affecting all of them…On the doctrine [of modularity] we either have to say that these devices are indeed impossible, or that what is going on from one step to the next inside the cover is not causation….” (2007: p.87)

I agree it’s implausible to say that such a device is impossible or to say that the processes within the mechanism are not causal.

Nancy Cartwright (2007) has proposed counterexamples involving machines to the principles. See Woodward (2008), Steel (2010), and Kuorikoski (2010) for responses to these

examples. But she also advances an interesting thought experiment:

“…[given modularity] we can see that it is impossible to build a bomb that cannot be defused. Nor can we make a deterministic device of this sort: the correct functioning of the mechanism requires that they operate in a vacuum; so we seal the whole device in a vacuum in such a way that we cannot penetrate the cover to affect one cause in the chain without affecting all of them…On the doctrine [of modularity] we either have to say that these devices are indeed impossible, or that what is going on from one step to the next inside the cover is not causation….” (2007: p.87)

I agree it’s implausible to say that such a device is impossible or to say that the processes within the mechanism are not causal.

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Here is a simple schematic mechanism to illustrate Cartwright’s thought experiment:

X = xY = f(X) X Y ZZ = g(Y)

Suppose that if we were to intervene on the variables X and Y we would disrupt the functional dependencies between X and Y and between Y and Z.

This is a problem precisely because it falsifies the interventionist counterfactuals that must be true if there are to be right causal relations: e.g.If an intervention were to change the value of X, the value of Y would change.

If an intervention were to change the value of Y, the value of Z would change.

Here is a simple schematic mechanism to illustrate Cartwright’s thought experiment:

X = xY = f(X) X Y ZZ = g(Y)

Suppose that if we were to intervene on the variables X and Y we would disrupt the functional dependencies between X and Y and between Y and Z.

This is a problem precisely because it falsifies the interventionist counterfactuals that must be true if there are to be right causal relations: e.g.If an intervention were to change the value of X, the value of Y would change.

If an intervention were to change the value of Y, the value of Z would change.

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Cartwright’s example might be described as a finkish mechanism. A finkish disposition is such that stimulating the disposition makes it go away. C.B. Martin’s example: a machine connected to a wire makes the wire cease to be live if it is touched by a conductor.

This is a special case of a finkish mechanism consisting of two equations:

S=0R=S S R

An intervention on S disrupts the disposition R = S and falsifies the counterfactual “If it were that S =1 it would be that R=1”.

Cartwright’s example might be described as a finkish mechanism. A finkish disposition is such that stimulating the disposition makes it go away. C.B. Martin’s example: a machine connected to a wire makes the wire cease to be live if it is touched by a conductor.

This is a special case of a finkish mechanism consisting of two equations:

S=0R=S S R

An intervention on S disrupts the disposition R = S and falsifies the counterfactual “If it were that S =1 it would be that R=1”.

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Still there is a connection between dispositions and counterfactuals which can be spelled out by noting the compatibility of these counterfactuals.

If an intervention were to change the value of S, then R = S would cease to hold.

If an intervention were to change the value of S while preserving R = S, then the value of R would change.

Of course, the second counterfactual employs the very concept of a disposition which is to be explained.

Morale: finkish dispositions frustrate a counterfactual explanation of dispositions.

Still there is a connection between dispositions and counterfactuals which can be spelled out by noting the compatibility of these counterfactuals.

If an intervention were to change the value of S, then R = S would cease to hold.

If an intervention were to change the value of S while preserving R = S, then the value of R would change.

Of course, the second counterfactual employs the very concept of a disposition which is to be explained.

Morale: finkish dispositions frustrate a counterfactual explanation of dispositions.

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It strikes me that Cartwright’s example of a finkish mechanism raises serious questions about the possibility of a counterfactual explication of causation. We saw that the following counterfactual in her example appears to be false:

If an intervention were to change the value of X, the value of Y would change.

Notice that Pearl’s account of intervention enables him to give the correct account of such counterfactuals.

“The simplest type of external intervention is one in which a single variable, say X, is forced to take on some fixed value x. Such an intervention amounts to lifting X from the influence of the old mechanism and placing it under the influence of a new mechanism that sets the value x while leaving all other mechanisms unperturbed.” (p.70)

In other words the modularity assumption is built into his account of an intervention.

It strikes me that Cartwright’s example of a finkish mechanism raises serious questions about the possibility of a counterfactual explication of causation. We saw that the following counterfactual in her example appears to be false:

If an intervention were to change the value of X, the value of Y would change.

Notice that Pearl’s account of intervention enables him to give the correct account of such counterfactuals.

“The simplest type of external intervention is one in which a single variable, say X, is forced to take on some fixed value x. Such an intervention amounts to lifting X from the influence of the old mechanism and placing it under the influence of a new mechanism that sets the value x while leaving all other mechanisms unperturbed.” (p.70)

In other words the modularity assumption is built into his account of an intervention.

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6. Criticisms of Modularity6. Criticisms of ModularityAccordingly, in an interventionist counterfactual such as

“If an intervention were to change the value of X, the value of Y would change”,

the built-in modularity assumption implies that all the functional dependences such as Y= f(X) and Z=g(Y) are preserved in evaluating the antecedent. This ensures that the counterfactual is correctly evaluated as true.

However, the cost of this feature of Pearl’s account is that the counterfactual cannot be used to explain the functional dependence Y=f(X)–precisely because it presupposes this very concept.

In contrast, Woodward is very careful in defining an intervention not to build in the requirement that an intervention on a variable X with respect to Y should preserve any functional dependence between X and Y. For this reason it seems that his account cannot deliver the correct interventionist counterfactuals that are required to handle Cartwright’s example of a finkish mechanism.

Accordingly, in an interventionist counterfactual such as

“If an intervention were to change the value of X, the value of Y would change”,

the built-in modularity assumption implies that all the functional dependences such as Y= f(X) and Z=g(Y) are preserved in evaluating the antecedent. This ensures that the counterfactual is correctly evaluated as true.

However, the cost of this feature of Pearl’s account is that the counterfactual cannot be used to explain the functional dependence Y=f(X)–precisely because it presupposes this very concept.

In contrast, Woodward is very careful in defining an intervention not to build in the requirement that an intervention on a variable X with respect to Y should preserve any functional dependence between X and Y. For this reason it seems that his account cannot deliver the correct interventionist counterfactuals that are required to handle Cartwright’s example of a finkish mechanism.

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This bears on another criticism that Cartwright makes of the modularity assumption. “The thing that is odd about the demand for modularity is where it locates the causal nexus. It is usual to suppose that the fact that C causes E depends on some relations between C and E. Modularity makes it depend on the relation between the causes of C and C: C cannot cause anything unless it itself is brought about in a very special way.” (2007: p.86)

Her point is that, on the interventionist account, if C causes E, there must be a possible intervention (with a distinctive causal history) that could change the value of C; and furthermore, it must do so without perturbing the other capacities of the system. This makes causation look to be an extrinsic phenomenon.

This bears on another criticism that Cartwright makes of the modularity assumption. “The thing that is odd about the demand for modularity is where it locates the causal nexus. It is usual to suppose that the fact that C causes E depends on some relations between C and E. Modularity makes it depend on the relation between the causes of C and C: C cannot cause anything unless it itself is brought about in a very special way.” (2007: p.86)

Her point is that, on the interventionist account, if C causes E, there must be a possible intervention (with a distinctive causal history) that could change the value of C; and furthermore, it must do so without perturbing the other capacities of the system. This makes causation look to be an extrinsic phenomenon.

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6.Criticisms of Modularity 6.Criticisms of Modularity Pearl’s version of interventionism can vindicate the intrinsicness

intuition: when a causal relation between C and E holds within a mechanism, it does so in virtue of the ‘intrinsic’ capacity linking C with E.

This is consistent with modularity. In testing the causal relation we have to produce C via a special causal pathway. But test conditions of a causal relation ≠ conditions of the relation’s obtaining.

This is to embrace a robust realism about capacities and causal relations. Whether a capacity linking C with E exists is independent of:

whether a human can actually intervene on C; whether an intervention, human or otherwise, actually occurs; whether an intervention is even physically possible.

This realism is perfectly consistent with Pearl’s interventionist account of causation but seemingly not Woodward’s.

Pearl’s version of interventionism can vindicate the intrinsicness intuition: when a causal relation between C and E holds within a mechanism, it does so in virtue of the ‘intrinsic’ capacity linking C with E.

This is consistent with modularity. In testing the causal relation we have to produce C via a special causal pathway. But test conditions of a causal relation ≠ conditions of the relation’s obtaining.

This is to embrace a robust realism about capacities and causal relations. Whether a capacity linking C with E exists is independent of:

whether a human can actually intervene on C; whether an intervention, human or otherwise, actually occurs; whether an intervention is even physically possible.

This realism is perfectly consistent with Pearl’s interventionist account of causation but seemingly not Woodward’s.

1111111 Mechanisms and Modularity: An Interventionist Approach [email protected].

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