In Daniel Dennett's remarkable book Darwin's Dangerous Idea he describes the Tower of Generate-and-Test; a brilliant conceptual model for the evolution of intelligence that has become known as Dennett's Tower.

Dennett's tower is set of conceptual creatures each one of which is successively more capable of reacting to, and hence surviving in, the world through having more sophisticated strategies for 'generating and testing' hypotheses about how to react. This doesn't just mean deciding what to do next when confronted with, for instance, a predator who just might be about to eat you (options: fight, flee, play dead, attempt to make friends, pray to your god, etc); by generate-and-test Dennett instead refers to how the creature figures out what the options are and then tries out each one. Read chapter 13 of Darwin's Dangerous Idea for the full account, but here's my very brief synopsis of the storeys of Dennett's tower, starting on the ground floor:

  • Darwinian creatures have only natural selection as the generate and test mechanism, so genetic mutation and selection is the only way that Darwinian creatures can adapt - individuals cannot.
  • Skinnerian creatures can learn but only by literally generating and testing all different possible actions then reinforcing the successful behaviour (which is ok providing you don't get eaten while testing a bad course of action).
  • Popperian creatures are Skinnerians that have the additional ability to internalise (imagine) the possible actions so that some (the bad ones) are discarded before they are tried out for real.
  • Gregorian creatures are tool makers including - importantly - mind tools like language, which means that individuals no longer have to generate-and-test all possible hypotheses since others have done so already and can pass on that knowledge.
  • Scientific creatures. Here Dennett proposes that a particular way of rigorously, collectively and publically testing hypotheses - namely the scientific method - is sufficiently powerful and distinct to merit a further floor of the tower. (I'm not sure that I agree, however that isn't important to the point I'm trying to make in this essay.)

Like the Tower of Hanoi each successive storey is smaller (a sub-set) of the storey below, thus all Skinnerian creatures are Darwinian, but only a sub-set of Darwinian creatures are Skinnerian and so on. I propose here another storey to the Tower, for what I want to call Walterian Creatures, after the pioneering neurophysiologist W. Grey Walter who, in 1948-49, invented his famous electro-mechanical robot tortoises, acknowledged by many roboticists as the precursors of what we now call Artificial Life. But before I explain what Walterians are I need to discuss Gregorians in more detail.

Gregorian creatures (named after British psychologist Richard Gregory) are tool makers, of both physical tools (like axes and wheels) and mind-tools (like language and mathematics), and Dennett suggests that these tools are 'intelligence amplifiers'. Certainly they give Gregorian creatures a significant advantage over merely Popperian creatures, because they have the benefit of the shared experience of others, expressed either through using the tools they have made or refined or, more directly, through their knowledge or instructions as spoken or written. I would argue that the most powerful intelligence amplifier so far created by one particular species of Gregorian-Scientific creature: man, is the computer, for with it we are able to simulate almost any reality we can imagine. Computer simulation is potent stuff, gedanken thought experiments are no longer doomed to remain flights of fancy and mathematical models need no longer remain dry abstractions. And one of the most remarkable kinds of computer simulation is of intelligence itself: Artificial Intelligence.

What if the tools made by Gregorian creatures take on a life of their own and become, in a sense, independent of the tool-makers? Embodied Artificial Intelligence (Artificial Life) has this potential. Walterian creatures are, I propose, smart tools that have learned to think, grown up and left the toolbox. Think of future intelligent robots (far more capable than the crude prototypes we can currently build) that might co-exist with humans in an extraordinary symbiosis.

The defining characteristic of Walterian creatures is that they are artificial. They've not only left the toolbox but crawled out of the gene pool. No longer bound by the common biochemistry of Earth's biota, yet sharing both the inheritance and evolutionary (albeit artificial) processes of their Darwinian ancestors. So what does this mean for Walterian creatures? Well, all of Walterian’s ancestors share the fact that however simple or sophisticated their strategies for hypothesising about possible actions those actions have to be undertaken by the self-same physical creatures that do the hypothesising. Ok, Gregorian-Scientific creatures can augment themselves with magnificent tools for compensating for their own sensory or physical limitations, like electron microscopes or manned spacecraft, or remotely operated robot space probes that act as sense extenders, but one thing Gregorian individuals cannot do is evolve themselves as part of the generate-and-test process. Consider this scenario. A future intelligent autonomous robot is exploring a planet about which very little is known. As part of its generate-and-test strategy this Walterian can in simulation fast-forward artificial genetic algorithms to evolve its own physical capabilities and then re-build parts of itself on-the-fly to best deal with the situation it has encountered. It could, for instance, artificially evolve and re-engineer itself the means to make best use of whatever energy sources are to hand. (It would be like you or I falling into a river and being able to artificially evolve and grow gills in less time than it takes to drown.)

Walterian creatures are, like Gregorians, able to share tools, knowledge and experience. They will be fully interconnected, so that any individual - subject only to the physical delays of the networking technology - can instantly seek information or resources from the shared Walterian artificial culture. However, unlike Gregorians, these individuals are capable of Lamarckian learning. Need a skill fast? If you’re a Walterian creature then, providing at least one other individual has already learned the skill and is either online or has previously uploaded that skill, then you simply download it. Walterian creatures would surely have profoundly different - and perhaps unimaginable by us merely Gregorian creatures - kinds of minds.

This essay is a revised version of a piece published on my blog in April 2007