I've read this book three or four times over the past two years, and every time I read it I become less and less impressed with it. Deacon's thesis is that life and the mind can be integrated into scientific theory by introducing the concept of constraint. In other words, constraints are the missing link, the last frontier that show us how matter turns into mind.
Deacon starts the book by noting that things like mind, meaning, morality, and purpose are all "missing" from the scientific worldview. We know that they are important, but science has nothing to say about them...yet. When you analyze the physical dynamics of the brain, you don't find experiences banging around inside the neurons. Instead, you find things like actions potentials, neurotransmitters, etc. So where are the experiences? Deacon says things like experiences, meanings and morals are "absential". We intuitively know they make a difference, but when we look at the physical world we never see them "there". The word "absence" is vitally important here, and it will come up again in the discussion of constraints. Deacon will ultimately try to tie two very different notions of absence together in order to provide an "explanation" of consciousness. What follows in my review is unavoidably complicated, which reflects the complex nature of the topic at hand. I'll attempt pull everything together at the end of the review to show where I think Deacon succeeds, and where he fails.
Enter the concept of constraint. What is a constraint? Constraint is a way of conceptualizing limitations, restrictions or absences(!). Railroad tracks constrain the movements of a train, government regulations restrict the actions of people and corporations, etc. Paradoxically, it turns out that constraints are the origin of regularity and patterns. To continue with the railroad metaphor, the railroad tracks limit the movements of the train to one particular path between two points. This limitation is actually vital to keeping the train on schedule. If the train was able to randomly take any path from point A to point B, then it would almost certainly never arrive on time. So the restriction imposed by the tracks is part of what enables the regularity of the train's schedule.
So how does this concept of constraint apply to physical systems and how does it help solve the problems of life and mind? It's actually a fairly complicated story (and Deacon does not explain it very well), but the story starts with the idea that every physical system has its own intrinsic dynamical tendencies. What does that mean? Take the familiar example of an ideal gas in isolation. Left to its own devices, such a system will tend toward a state of greater entropy until it reaches the state of maximum entropy. This is the system's intrinsic dynamical pattern, meaning that it does this all on its own without any influence from outside sources. This is actually quite interesting when you think about it. This means that change is a fundamental aspect of our physical universe, and all physical systems exhibit a particular "type" of change that is intrinsic to the type of thing they are. This is analogous to Aristotle's concept of a formal cause, meaning that the "form" or "type" of the object determines how it behaves in isolation. Deacon coins the term "homeodynamics" to describe the dynamics of systems in isolation, and uses the word "orthograde" to describe changes that are intrinsic to what a system is.
In the real world, physical systems are almost never in isolation. When systems interact they do "work" on one another (via the application of force). The types of "work" that systems do on one another is determined by their "orthograde" tendencies. In other words, a system's intrinsic dynamics is what allows it to apply force to another system. When work is applied to a system from the outside it imposes "constraints" on that system. The forces imposed from the outside actually counteract the system's natural tendencies and force it to behave in ways that may not be natural to its homeodynamics. Deacon coins the term "contrgrade" to describe this type of change. Thus forces cause constraints to arise in a system and, as we'll see, these constraints can lead to structure and regularity.
In the end, the application of constraints are a function of work done on the system by the application of external forces. The forces place constraints on the system's natural dynamical tendencies. Sometimes, so much constraint is imposed that the resulting dynamics is highly structured and redundant. Think of highly patterned structures such as crystals, rocks, etc. These structures are able to temporarily defy the second law of thermodynamics because of the constraints they exhibit. These constraints originated from some application of force (and a resultant transfer of energy) in the distant or recent past. That energy source is no longer present, but the structure remains until it finally degrades. Deacon coins the term "morphodynamic" to describe highly structured natural systems that result from high levels of constraint. Morphodynamics can only arise via contragade change.
One interesting thing to note is that morphodynamic systems exhibit their own form of "orthograde" dynamics that is quite distinct from the "orthograde" dynamics of the systems out of which they are composed. In this way, Deacon has constructed a "ladder" of emergence. The logic goes something like this:
1. System A is in isolation and exhibits its own natural orthograde behavior.
2. System B is in isolation and exhibits its own natural orthograde behavior.
3. System A and system B collide.
4. System B applies force to system A and vice versa.
5. The force applied by system B imposes limits/constraints to system A, and vice versa.
6. These limits/constraints impose restrictions on system A's degrees of freedom, and vice versa. This results in contragrade behavior.
7. These limits/constraints may dramatically increase the regularity of system A's behavior (think of the train track metaphor), thus leading to morphodynamics.
8. The result of the interaction of system A and system B is, thus, itself a new system (system C). The constraints imposed on A by B (and vice versa) are intrinsic to system C and define system C's orthograde dynamics.
9. So now we have a new system (system C) that is in isolation and exhibits its own natural orthograde behavior.
This is the logic of emergence up to the level of morphodynamics. It explains how ordered behavior (system C) can emerge out of disordered behavior (Systems A and B). System C is a novel system with its own orthograde dynamics. System C's existence is born of constraints. System C can go on to have interactions with other systems for as long as it persists, and in the process it will transfer constraints to create yet another new system. System C's behavior cannot be "reduced" to the behavior of systems A and B because the difference is defined by what was constrained (i.e. what is now absent). Thus emergent behavior is not "something more" but rather "something less" that what was originally present. So although system C is dynamically irreducible, it is nonetheless nothing more than physical processes.
This is actually an incredibly brilliant solution to the problem of emergent behavior. However, I have since discovered that the idea is not entirely original to Deacon (though his is certainly the most thorough and explicit explication of it). While his account of it is certainly idiosyncratic, there have been at least a couple of other authors who said very similar things before him. It is not clear whether he was aware of those authors' work before he published his, but he certainly did not reference those authors, so let's hope he was just unaware.
In any event, the same trick can be used to make the jump from non-life to life, except the explanation is now even more complex. Deacon calls the next level of emergence "teleodynamics". We already saw how morphodynamics is the emergence of order out of disorder. However, life is more than just order. It also has a teleological aspect to it. In other words, not only does it exhibit order, but it takes actions in order to preserve that order. life is fundamentally self-preserving, and it will even make changes to itself (i.e. adaptations) in order to accomplish self-preservation.
I'm not going to go into the details of how the jump from morphodynamics to teleodynamics is made. The summary is that two or more morphodynamics systems become "coupled" in such a way that the dynamics of each system are symbiotic and mutually supportive to the continuance of the interaction. This review is already too long so I'll say no more on the topic, except to say that I found the account to be quite convincing. I would say that Deacon has succeeded in defining life in the abstract and showing in detail how it emerges from non-life, even if he wasn't the first person to offer such an account.
Deacon then goes on to recast various other concepts in terms of constraint. He presents constraint-based treatments of work, information, signification/semiosis, evolution, sentience and, finally, consciousness. It is particularly when Deacon tries to make the jump from semiosis to sentience that his account starts to flounder. This is where his fundamental equivocation comes into play. Remember that Deacon defined things like consciousness, morality and meaning as "absential" phenomena, implying that they are not present when we examine physical systems at any level. We know they make a difference but we never actually "see" them anywhere in the physical world. Constraints are defined very similarly. A constraint defines what is not present in a physical system but could have been. So constraints define what is absent (but could have been present) in physical systems.
Deacon's grand thesis, then, is that consciousness, morality and meaning are literally constraints in physical systems. If we are to believe Deacon, then consciousness is defined literally as a set of physical states that could have occurred, but did not. This is an equivocation on the word "absential" as he used it in reference to consciousness, meaning and morality earlier in his book, and it's not clear how reducing consciousness to constraints solves anything. In fact, it is a bit disingenuous of Deacon to make this claim, especially considering how he spent the first part of the book taking materialists to task for trying to reduce consciousness to physical processes. But Deacon is in fact attempting to perform the same reduction, but instead of targeting "actual" physical processes, he is targeting "potential" physical processes. If the reduction doesn't work for actual physical states, then why should we accept that it would work for physical states that are merely potential?
So Deacon's work is both brilliant and frustrating. I do think that Deacon (and others before him) has found the key to unlocking the riddle of emergence, and for that alone I would recommend this book to anyone. However, as far as science is concerned, emergence is intrinsically about behavior. What emerges from the interaction of systems is novel behavior, nothing more nothing less. So if you are inclined to believe that consciousness and sentience can be explained in terms of the behavior, then you will probably find Deacon's argument convincing. However if you think that there is an aspect of consciousness that is intrinsically and irreducibly subjective, then you are not likely to be satisfied.
Ultimately, everyone has to decide for themselves whether they can accept or understand consciousness as an objective phenomenon. The smell of roses, the taste of blueberry cobbler, the radiant gleam of the setting sun: you either accept that these things are in some sense irreducibly "real" in their own right, or you don't. Deacon seems to have placed himself on the side of those who don't, and as such, I doubt this book will be seen by all as the final word on what consciousness is.