Recursion and the human mind

Although not completely natural, recursive thinking is uniquely a human trait, and it has been suggested that it is one of the characteristics that distinguishes human language from other forms of animal communication [1]. Corballis [2] suggests that recursion gives humans the ability to “mentally travel in time”. This is based on the idea of episodic memory, which refers to particular episodes of life that can be relived in our minds. This differs from semantic memory which is the storehouse of knowledge. Episodic memories are considered to be recursive, as they involve making mental references to an earlier mental self. Canadian neuroscientist Endel Tulving [3] suggests that recovery of semantic memories requires noetic awareness, which is simply knowing. Conversely, the recovery of episodic memories requires autonoetic awareness, or self-knowing. So episodic memories are recursive because they involves making references to an earlier mental self. Therefore the recursiveness of thought is intrinsically tied to the notion of self, or as Corballis puts it “not merely knowing that one is a physical object , but knowing that one knows, or knowing that one has mental states”. 

This concept is fundamental to the evolution of technology. Tool use is by no means unique to humans, however humans are the only animals that have been observed using a tool to make a tool, which is then used to make a tool [4]. A blacksmiths hammer is used to craft a hammer, which is then used to craft a hammer, etc. Development of tools requires travel into the past to explore previous designs of hammer, as  well as the future to envision new designs.

However the identification of recursive processes used on a daily basis is elusive, as they are often attributed to other methods [5]. Imagine if you will, a tree that has been felled. The act of de-limbing the tree (i.e. cutting off all its branches to leave just the trunk) could be thought of as a recursive process.

       if (tree has limbs)
          cut off lowest limb

The problem is that this process is just as easily described in an iterative manner:

       while (tree has limbs)
          cut off lowest limb

And in reality, this is how the process is perceived by humans. Humans don’t actively think about recursion. Sometimes we may actively use recursion to solve problems, but likely don’t realize it.

[1] Hauser, M.D., Chomsky, N., Fitch, W.T., “The faculty of language: What it is, who has it, and how did it evolve?” Science, 298, pp. 1569-1579 (2002)[2] Corballis, M., “The uniqueness of human recursive thinking: the ability to think about thinking may be the critical attribute that distinguishes us from all other species”, American Scientist, 95(3), pp.240-248 (2007)
[3] Tulving, E., Elements of Episodic Memory, London, Oxford University Press (1983)
[4] Beck, B.B., Animal Tool Behavior: The Use and Manufacture of Tools by Animals, Garland STPM Press, New York (1980)
[5] Vitale, B., “Elusive recursion: A trip in recursive land”, New Ideas in Psychology, 7(3), pp.253-276 (1989)

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