Symbols, Language and Human Uniqueness

David Premack wrote an interesting perspective piece in the January 2004 issue of Science concerning the uniqueness of human cognition, as distinct from other primates, with regard to our use of symbols. Before debating his perspective, I'll review it in detail.

According to Premack, six symbol systems are in use by humans: the genetic code, spoken language, written language, arabic numerals, music notation, labanotation (a choreography coding scheme, oddly). Premack suggests that the first two "evolved" whereas the last four were invented. The rest of his article concerns the extent to which our use of symbols is unique in the animal kingdom. The authors identifies six ways in which human symbol use is unique, discussed in brief below:

• Voluntary Control of Motor Behavior. Premack argues that because both vocalization and facial expression are largely involuntary in the chimpanzee, they are incapable of developing a symbol system like speech or sign language.
• Imitation. Because chimpanzees can only imitate an actor's actions on an object, but not the actions in the absence of the object that was acted upon, Premack suggests that language cannot evolve.
• Teaching. Premack claims that teaching behaviors are strictly human, defining teaching as "reverse imitation" - in which a model actor observes and corrects an imitator.
  
• Theory of Mind. Chimps can ascribe goals to others' actions, but Premack suggests these attributions are limited in recursion (i.e., no "I think you thought he would have thought that.") Premack states that because recursion is a necessary component of human language, and because all other animals lack recursion, they cannot possibly evolve human language.
• Grammar. Not only do chimps use nonrecursive grammars, they also use only words that are grounded in sensory experience - according to Premack, all attempts have failed to train chimps to use words with meanings grounded in metaphor rather than sensory experience.
• Intelligence. Here Premack suggests that the uniquely human characteristics of language are supported by human intelligence. Our capacity to flexibly recombine pieces of sensory experience supports language, while the relative lack of such flexibility in other animals precludes them from using human-language like symbol systems.
  

Unfortunately, this cross-species comparison of symbolic systems seems flawed for a few reasons:

1. There is no established test for what makes a given action voluntary, and so we cannot know that animal motor acts are truly involuntary.
   
2. Voluntariness is irrelevant to the question of language. For example, if I am drunk out of my mind, and I involuntarily blurt out a recursive, grammatical and metaphorical verbalization, it is still human language, despite being involuntary.
   
3. Imitation in the absence of an acted-upon object is also irrelevant to the question of language. Language is purely symbolic, meaning that it does not directly act upon anything, and so it's unclear why the physical circumstances of imitation should be so important.
   
4. If one insists that physical imitation skills are critically relevant, Premack's claims still contradict evidence reviewed here suggesting that primates are capable of symbolic play without human instruction. And then there's some remarkable work from the Brazilian rainforest showing how primitive some human symbol use is.
5. Teaching (even using Premack's narrow definition) is not a strictly human behavior: consider this evidence, reviewed by John Hawks, that meerkats modify the behavior of their errant meerkat pupils. Yet, as far as we know, meerkats do not have a human-like symbolic system. Thus, teaching of this sort seems irrelevant to the evolution of language.
   

The other sections have their flaws as well, but none as critical as those laid out above. Still other vagaries are more philosophical in nature, and often completely immaterial to Premack's point: for example, how to distinguish symbols that evolved from those that were invented?

In summary, it seems like Premack has simply asked the wrong question. Instead of asking "is language the key to human intelligence?" (which forms the title of his piece), it may be more productive to ask "is intelligence the key to human language?"