Hart's 'review' is not so much a review as a meandering, bile-filled rant wrapped up in obscurantist terminology. I find it very hard to extract an actual argument from it, mostly because of the lack of a clear line of thought. But to the degree that there is a core to what he seems to be trying to say, he claims that (a) language and (b) our mind or consciousness are irreducibly complex. To quote only the perhaps most immediately relevant parts:
There is simply no causal narrative -- and probably never can be one -- capable of uniting the phenomenologically discontinuous regions of "third-person" electrochemical brain events and "first-person" experiences, nor any imaginable science logically capable of crossing that absolute qualitative chasm.And so on, believe it or not, in the same tone for a total of more than 5,400 words. I cannot for one moment image writing a book review of even half of that length.
Then there is the irreducible unity of apprehension, without which there could be no coherent perception of anything at all, not even disjunctions within experience. As Kant among others realized, this probably poses an insuperable difficulty for materialism. It is a unity that certainly cannot be reduced to some executive material faculty of the brain, as this would itself be a composite reality in need of unification by some still-more-original faculty, and so on forever, and whatever lay at the "end" of that infinite regress would already have to possess an inexplicable prior understanding of the diversity of experience that it organizes. For, even if we accept that the mind merely represents the world to itself under an assortment of convenient fictions, this would involve a translation of sense data into specific perceptions and meanings; and translation requires a competence transcending the difference between the original "text" and its rendition.
This problem, moreover, points toward the far more capacious and crucial one of mental intentionality as such -- the mind's pure directedness (such that its thoughts are about things), its interpretation of sense experience under determinate aspects and meanings, its movement toward particular ends, its power to act according to rationales that would appear nowhere within any inventory of antecedent physical causes. All of these indicate an irreducibly teleological structure to thought incongruous with a closed physical order supposedly devoid of purposive causality.
Similarly, there is the problem of the semantic and syntactic structure of rational thought, whose logically determined sequences seem impossible to reconcile with any supposed sufficiency of the continuous stream of physical causes occurring in the brain.
[...]
In every case, most of his argument consists in a small set of simple logical errors. The most conspicuous is one I think of as the "pleonastic fallacy": the attempt to explain away an absolute qualitative difference -- such as that between third-person physical events and first-person consciousness -- by positing an indefinite number of minute quantitative steps, genetic or structural, supposedly sufficient to span the interval. Somewhere in the depths of phylogenic history something happened, and somewhere in the depths of our neurological machinery something happens, and both those somethings have accomplished within us an inversion of brute, mindless, physical causality into, at the very least, the appearance of unified intentional consciousness.
[...]
Everything in nature must for him be the result of a vast sequence of tiny steps. This is a fair enough position, but the burden of any narrative of emergence framed in those terms is that the stochastic logic of the tale must be guarded with untiring vigilance against any intrusion by "higher causes." But, where consciousness is concerned, this may very well be an impossible task.
[...]
So, for Dennett, language must have arisen out of social practices of communication, rooted in basic animal gestures and sounds in an initially accidental association with features of the environment. Only afterward could these elements have become words, spreading and combining and developing into complex structures of reference. There must then, he assumes, have been "proto-languages" that have since died away, liminal systems of communication filling up the interval between animal vocalizations and human semiotic and syntactic capacities.
Unfortunately, this simply cannot be. There is no trace in nature even of primitive languages, let alone proto-languages; all languages possess a full hierarchy of grammatical constraints and powers. And this is not merely an argument from absence, like the missing fossils of all those dragons or unicorns that must have once existed. It is logically impossible even to reverse-engineer anything that would qualify as a proto-language. Every attempt to do so will turn out secretly to rely on the syntactic and semiotic functions of fully developed human language. But Dennett is quite right about how immense an evolutionary saltation the sudden emergence of language would really be. Even the simple algorithm of Merge involves, for instance, a crucial disjunction between what linguists call "structural proximity" and "linear proximity" -- between, that is, a hypotactic or grammatical connection between parts of a sentence, regardless of their spatial and temporal proximity to one another, and the simple sequential ordering of signifiers in that sentence. Without such a disjunction, nothing resembling linguistic practice is possible; yet that disjunction can itself exist nowhere except in language.
Anyway, this is pretty much the same argument as always. A creationist would say, look, the eye is really complicated. Half an eye would not work, so how could an eye have evolved? And Hart says, look, language / consciousness is really complicated. There is no half-language or half-consciousness, so how could they have evolved?
Now unfortunately for the creationist, nature abounds in half-eyes, so to say. Extant animals show everything from single light-sensitive cells across groups of such cells arranged in a little depression of the skin across ocelli to vertebrate style camera eyes of varying degrees of sophistication. Hart has it a bit easier in that our ancestors who would have had half-languages are gone, so he finds it possible to claim that such intermediates are unthinkable.
It is less clear to me how the same claim can reasonably be made about consciousness given the obvious progression in mental capability from worms to chimpanzees, but let's assume for present purposes that there is a vast gulf between us and any other species on the planet. The problem is still that Hart's position falls apart the moment somebody vaguely gestures towards children. This idea is not original to me, indeed one of the commenters on the otherwise mostly distressingly woolly Crooked Timber thread soon made the same point.
Note what I am not saying. I am not saying that Dennett is right about everything he wrote in his book. I have not read it, nor do I have any intention of doing so anytime soon. More to the present point, I am not saying that human ontogeny recapitulates evolutionary history. I am not saying that a six month old human's mind is a very accurate model of a rodent mind or suchlike.
But it is nonetheless obviously and demonstrably the case that every human starts as one cell, clearly without a consciousness, and (if healthy and unharmed) develops gradually into an extremely complex being with what Hart calls consciousness*. Yes, somewhere during ontogeny "something happens", and as far as I can tell most of us would accept that it is gradual. This indicates that gradual evolution can well be assumed to have produced this outcome, no magic involved.
Likewise it is obviously and demonstrably the case that young humans do not suddenly acquire "a full hierarchy of grammatical constraints and powers" in one go. Has Hart actually ever communicated with a toddler? We acquire our language competence gradually, and again this indicates that humans can well be assumed to have acquired language competence gradually over the course of their evolution.
For language in particular it is indeed trivial to visualise how even very rudimentary language immediately confers an advantage. Forget grammar; merely being able to grunt a few words meaning "danger", "food", and "shelter" while pointing into the distance would have served our ancestors quite well at some point. Next they may have come up with a few verbs. Even without tenses and suchlike "you wait; me go" will convey useful information, and of course lots of tourists successfully communicate at this level in a local language.
Again, I do not know how good Dennett's book is. Nor do I claim to have all the answers. But sometimes it is quite easy to tell that a given answer must be wrong even if you don't know what the right answer is. And it is really fascinating to read with what a tone of condescension Hart argues that there cannot be a gradualist explanation for capabilities that are demonstrably acquired gradually during human ontogenetic development.
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*) Another point potentially to be made here is that certain philosophers of mind frustratingly take terms that have been invented to give a name to an observed phenomenon and then mystify them to the point where they cannot imagine a non-magic explanation for the phenomenon.
Consciousness is a perfect example. When our ancestors came up with that word they would have done so to describe the difference between a sleeping, knocked-out, drugged or dead person on one side and an awake and aware person on the other. They would then at some point have observed that while for example a beetle may be kind of awake and aware it does not appear to have the same awareness of itself (~consciousness) as a human. Finally a certain type of philosopher of mind comes in, picks up the term consciousness, reinterprets this label slapped on an observed distinction into a mysterious substance sitting in human heads, and claims that such a mysterious substance cannot be explained scientifically.
I would take one step back and say, no, actually this is straightforward in principle (if not necessarily easy in practice): that person over there is sleeping, just study how the brain processes differ from when they are awake and you have your explanation. Of course this is not what the philosopher means, but it is what consciousness means! The same goes for qualia, experience, apprehension, intentionality, and a whole host of other buzzwords used by Hart. They all have been created to describe observed distinctions, and being observable they can be studied using empirical science.
Luckily most scientists and philosophers understand the difference between things and processes, the concept of emergent processes, and the fallacies of composition, division and reification. (The last of these seems particularly relevant to how Hart discusses the terms mentioned in the previous paragraph.)
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