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Susan Hurley’s Vehicle Externalism of Consciousness

Philosopher Susan Hurley (1954-2007) explores how consciousness can be understood from an externalist perspective, in her book Consciousness in Action (Hurley 2002).[1] She is skeptical of a picture of the mind in which the mind is localized inside the head. She calls it the input-output picture, where perception is input into the mind and action is its output.

Hurley notes that perception and action are seen as mere buffer zones—nothing mental. Mental life lies between those buffers. The mind is an executive agent, which gets input through perception and orders output action through motor commands. But the mind can also drive output action through the world to cause new perceptual input.

Hurley gives the example of walking around a corner to perceive something different (Hurley 2002, p. 10). In such a case, the mind takes instrumental action to gain perceptual vantage points.

Hurley further notes how a perceptual change could result simply from output. If a person with paralyzed eye muscles tries to look to the left, the world appears to jump left. Perception changes because of mere motor intention. That perception changes without a change in input is, however, a challenge to the input-output picture.

Hurley also attributes a further assumption to the input-output picture: that perception, the mind, and action map to corresponding subpersonal neural processes. The mind is thought of as being on a personal level, and to explain it, we must map it to subpersonal neural processes.

Hurley adopts a critical perspective on this one-to-one relation between the mind and the brain, noting that multiple-personality patients may have multiple conscious beings associated with anatomically intact brains. Hurley concludes that neural unity and unity of consciousness don’t necessarily go hand in hand.[2]

She also asks us to consider a hypothetical case with two subjects lacking a corpus callosum. Congenitally acallosal subjects are born without a corpus callosum. But they behave as normal subjects even under the scrutiny of experimental testing.

When the corpus callosum is severed or absent within one body, we may have either a commissurotomy patient, who seems to support separate centers of consciousness, or a callosal agenesis patient (someone born without a corpus callosum). Callosal agenesis patients, or acallosals, typically pass almost all the experimental tests of unity that commissurotomy patients fail, including under conditions involving fixation. Their actions argue for a unified consciousness, even in experimental conditions and despite their similarity in gross neuroanatomical structure to commissurotomy patients. (Hurley 2002, p. 189)

In Hurley’s view, we should think of congenital acallosals as achieving the integration of consciousness that commissurotomy patients lack. How could this be? Hurley ponders the hypothetical development of two acallosals and how consciousness evolved in them. In one, integration of consciousness was achieved through alternative neural pathways. I will call him Mr. Internal. In Hurley’s view, if Mr. Internal behaves like anyone else, then he has achieved unity of consciousness. We should not be chauvinist about callosal fibers and say that only they can support hemispheric integration that is adequate for unity of consciousness. Mr. Internal has deployed other fibers for the same function. Now let us examine the other acallosal. I will call him Mr. External. He has achieved integration of consciousness through extracranial means, such as access movements and cross-cuing. Access movements serve to provide both hemispheres with the same visual information—for example, by looking from side to side or turning back and forth. Cross-cuing refers to behavioral interhemispheric information transfer. Hurley refers us to neuroscientist Joseph Bogen (1926-2005), who gives a list of examples.[3] One is of a blindfolded commissurotomy patient who moves objects between his hands to give both hemispheres access to them:

When a paper clip was placed in his left hand, he was completely at a loss. He then reached over to take it with his right hand and immediately and correctly named it. (Here, transfer from left to right hand made sufficient information available to the speaking left hemisphere.) (Bogen 1990, p. 218)

Hurley also gives an example of using facial expressions for cross-cuing—one hemisphere may make a facial expression that the other detects. Moreover, she suggests that cross-cuing should not be thought of deliberatively. Mr. External is not thinking about his cross-cuing—it happens for him automatically because he lived from the start without a corpus callosum and acquired deeply ingrained, automatic cuing and access habits. In such a situation, we should not think of the differences between Mr. Internal and External as more than superficial. Mr. Internal relies on internal mechanisms and Mr. External on external mechanisms, but they are functionally equivalent. Both manage to integrate consciousness in adequate ways. We should now see that consciousness could extend extracranially through mechanical means, in much the same way as consciousness extends between our hemispheres through the corpus callosum—it is a matter of replacing one set of mechanical, causal interactions with others that function reliably and for the same purposes:

For acallosals, to the extent either external or internal mechanisms of integration function reliably, there is no reason not to regard them as part of the vehicles of co-conscious contents and of a unified consciousness. (Hurley 2002, p. 191)

However, she also speculates that a commissurotomy patient might come to reach the same stage of external integration, given some time:

External mechanisms of integration acquire for her [a hypothetical subject with a commissurotomy] the same status as they have for our hypothetical acallosal: that of a subpersonal basis for the unity of consciousness, as opposed to a means of communication between separate consciousnesses. (Hurley 2002, p. 192)

If it is the case that consciousness can extend over external vehicles, then we need to redraw the input-output picture so it reflects the more complex situation. We also need to say something about what it is that unifies consciousness. Hurley suggests that the right way to understand the mind, consciousness, perception, and action involves openness to complexity and variability at the subpersonal level. The mind is dependent on a more complex subpersonal level than on the input-output picture. This subpersonal level that carries the mind—the level with the vehicles of all mental—is to be understood as being composed of the central nervous system (CNS), the body, and the environment as they are connected through complex looping structures where inputs and outputs are interdependent. She calls this subpersonal level a dynamic singularity.

Hurley also finds a corresponding interdependence at the personal level between perception and action. If an agent understands the interdependence between what the agent does and perceives at the personal level, then the agent has a perspective that allows for distinguishing the agent from the environment. Having such perspective is central to the unity of consciousness and to self-consciousness for Hurley.

Anyone who has a unified consciousness must also be rational, according to Hurley. It is possible to have a unified consciousness and be irrational to some degree, but not to a any degree. At some threshold of irrationality, unity of consciousness breaks down because of conflicting desires, beliefs, and actions.

There can be no sharp distinctions between the mind, self, body, and world if we take the dynamic singularity seriously. The mind and self are not confined to the head but are embedded also in the body and world, carried through feedback loops of the dynamic singularity.

Hurley suggests that being a living thing with perspective and access to content might be sufficient for consciousness:

Neither perspective nor access to content seems to be sufficient for consciousness. It seems that a robot could have both, yet be a “zombie,” without conscious states. Could adding in conceptual abilities keep such zombie worries at bay? Or, could it be sufficient for consciousness that a living thing has both perspective and access to content? (Hurley 2002, p. 17)

She returns to the question again in the fourth chapter:

Could it be sufficient for consciousness if a living thing has both perspective and access to content? (Hurley 2002, p. 161)

She also closes the chapter with it:

But could it be sufficient for consciousness for a living thing to have perspective and access to contents? (Hurley 2002, p. 163)

Then she suspends the question:

This connection is not pursued further here, but is left as a project for further work, as is any connection there may be between life, the criteria for unity, and consciousness. (Hurley 2002, p. 217)

We are left with the suggestion that life makes it possible to have conscious experiences. If we try to work out the suggestion by looking for clues about how Hurley might think about life, we find a reference to a book by artificial intelligence (AI) researcher Margaret Boden (1936-).[4] The first sentence in that book states, “Artificial Life (A-Life) uses informational concepts and computer modeling to study life in general, and terrestrial life in particular.” Such an approach to life would be consistent with Hurley’s general allegiance to formal approaches to the mind. She describes the plots of consciousness in action in the following way in a section of the book designed to inform the reader of her general approach, titled “Affinities and Implications”:

The sketched subplots have clear affinities with developments in connectionism, dynamic systems theory, and artificial life, as well as evident antecedents in cybernetics. (Hurley

2002, p. 22)

No other affinities are mentioned. On the whole, as Hurley describes her approach to the mind, it bottoms out in a formal account sketched at an abstract level. There is little in Hurley’s book that points to anything else in solving the problems of life and consciousness. Brains are only part of the dynamic singularity as functionally specified. If I had something else in my head that functioned like my brain, then there would be no difference on the personal level.

Hurley considers the question of life important for understanding consciousness. Yet she doesn’t fully tackle it. We got an account of how consciousness could extend over external vehicles as she considered the case of a person born without a corpus callosum—a congenital acallosal subject. In her view, such a person would incorporate external vehicles to achieve integration of consciousness. So consciousness would span over extracranial entities. The congenital acallosal is a clear example for her of how we can illustrate the workings of external vehicles of consciousness. Her project, as she describes it,[5] is “preliminary ground work”—a “thin edge opera- tion”—and she states that “how far we can push the wedge is a further question.” The thin edge of the wedge is to convincingly demonstrate that consciousness can span over external vehicles and make it plausible that this could happen in everyday life. She also hints at how the wedge could be pushed further by raising the question several times about what the necessary and sufficient conditions could be for attributing the presence of consciousness. As we have seen, each time Hurley raises this question, she does it in relation to the question of life. We are to think of life as holding the key to understanding consciousness, and it appears that it is in this direction that the wedge is to be pushed further.

  • [1] Hurley is explicit about this in response to a comparison between Consciousness in Action andClarke and Chalmers’s The Extended Mind: “I want to extend the consideration of more radicalexternalism to consciousness as well as thought”; see the online discussion with Timo Jarvilehto at
  • [2] Hurley states, “A unified consciousness need not depend on neuroanatomical unity, and neuroanatomical unity doesn’t rule out splits in consciousness” and “When the corpus callosum is intactwithin one body, we may have either a normal person with presumably normal unity of consciousness, or we may have a multiple personality patient who seems to support separate centers ofconsciousness” Hurley (2002), pp. 18 and 188, respectively.
  • [3] Bogen explains cross-cuing in the following way: “ ‘Crosscueing’ means that one hemisphereinitiates a bodily behavior which can provide information to the other hemisphere” Bogen (1990).
  • [4] Hurley (2002, p. 162) states, “No account of life is given here: that is another substantive question” and refers the reader to Boden (1996) The Philosophy of Artificial Life, a work by artificialintelligence researcher Margaret Boden. The first sentence of that book reads, “Artificial Life(A-Life) uses informational concepts and computer modelling to study life in general, and terrestrial life in particular.”
  • [5] See the replies by Hurley to comments on her book by Timo Jarvilehto at from a book symposium on Hurley (2002).
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