Both Darwin and Penn et al. are correct. There are enormous differences between human and animal minds, but enormous differences can arise from seemingly subtle changes in mental function. An example is the use of motor-sensory feedback to elaborate human thinking, based on plans that can circulate through the human brain repeatedly.
For the last century, the stimulus–response link has dominated psychology, connecting environment and behavior. The job of psychology was to explain what happened between stimulus and response, and not much else. In behaviorism this orientation was obvious, but modern cognitive psychology retains a similar orientation. The cognitive psychologist's theories allow additional boxes, representing internal processes, interposed between stimulus and response, but the diagrams always have a stimulus at one end and a response at the other. The approach of “new connectionism” is similar at a neural level of modeling, with hidden units allowed.
This psychology is incomplete in that the motivation for activating a particular block-diagram model remains unspecified. That is, we may understand how visual information from an object is processed, but the theory does not address why the subject was looking at the object in the first place. The act of looking was motivated by a plan, an internally held image of an intended achievement (Miller et al. Reference Miller, Galanter and Pribram1960). The plan is defined more broadly here than in its everyday sense, to mean a scheme that can control a sequence of actions to achieve a goal. Because plans motivate behavior, they become the keys to understanding control of behavior (Shallice Reference Shallice, Pope and Singer1978).
The power of plans is that they allow an organism to escape the contingencies of its immediate surroundings, to be controlled instead by its own needs in the longer term. A simple animal responds to its environment and to its internal states without need for plans. The fly does not make a decision to feed: When food is at hand in the environment and internal receptors are in the right states, feeding simply happens. More complex organisms can store many plans simultaneously, executing one while holding others in abeyance. Humans typically have hundreds of plans, from small, immediate ones, such as hammering a nail, to large ones, such as earning a college degree. Large-scale plans contain a hierarchy of smaller-scale plans. As a plan is executed, a single goal or idea is unpacked into a series of actions. Plans can also be interpreted in terms of motivation; modern conceptions of motivation include the pathway from motivating influences such as instinctive needs or selective attention, through plans, to action (Heckhausen Reference Heckhausen1991). The plan becomes the path from motivation to action.
Organizing behavior by plans requires neurological machinery to support the planning function and its ancillary needs. There must be neurological devices to (1) make plans, (2) store them, (3) execute them, and (4) monitor them (Bridgeman Reference Bridgeman1986; Reference Bridgeman1988). In order to control behavior, the currently active plan must have access to memory and attention. It must link these functions with perception to guide action according to a combination of the internally held plan and the external realities of the perceptual world. The plan-executing module takes a parallel, simultaneous idea and converts it into a sequence of serial behaviors or subplans. The mechanism exists in all primates, perhaps in all mammals. Chimpanzees can plan a behavioral sequence of several steps, such as piling boxes to reach a banana suspended overhead (Koehler 1925/1959). Even dogs can solve detour problems requiring them to move away from a reward in order to reach it eventually.
During human evolution this plan-executing module became more complex and loomed ever more important in mental life. At one point, I suggest, it began to be pressed into service for organizing communication as well as action. A single idea is unpacked into a series of words, and those in turn into phonemes, in the same way that any other plan is elaborated. In both the execution of actions on the world and the execution of communications, an ordered sequence of internally organized acts replaces the environmentally released behaviors of simpler animals (Bridgeman Reference Bridgeman1992).
Analogously, speech understanding uses another existing module. It normally monitors the progress of plans, taking a sequence of events and packing them into an idea. Language could evolve quickly, on an evolutionary time scale, because it was made mostly out of old parts. The sequencing and comprehension mechanisms were already developed for the execution and monitoring of actions, respectively. Only the articulatory apparatus and perhaps a specialized grammatical ordering system (Bickerton Reference Bickerton1984) had to be added.
Do planning and language share the same mechanism, or did a new planning mechanism evolve to specialize in language? Evolution of new functions often begins by doubling of genes, resulting in two copies of a morphological feature. Many cereal grains, for example, developed in this way. One of the copies is then free to evolve into something else. The open and flexible property of the planning mechanism, however, allows language to be added to its functions with little or no change. The mechanism already had the capacity to handle many plans simultaneously, to organize each one, to prioritize them, and to handle plans for different kinds of actions.
Once an utterance occurs, it is perceived like any other event. Perception of nonlinguistic utterances puts us in touch with emotions, whereas perception of our own language puts us in contact with our ideas. An animal is limited to awareness of its actions and emotional states through this mechanism, but for humans the same mechanism allows the conclusions of a thought process to be articulated by mechanisms that evolved for communication. Those conclusions re-enter the brain by mechanisms that evolved for understanding the communications of others, and the thought can make another cycle through the brain. The critical event is the planning, not the perception, because in human brains the sensory-to-motor feedback can be internalized. With this change, not in the mechanism of motor-sensory feedback, but rather, in its content, humans can think, whereas other animals are limited to whatever ideas pop into their heads.
