Mind, Body, World- Foundations of Cognitive Science, 2013a

5.6 Horizontal Layers of Control
Classical cognitive science usually assumes that the primary purpose of cognition is
planning (Anderson, 1983; Newell, 1990); this planning is used to mediate perception
and action. As a result, classical theories take the form of the sense-think-act
cycle (Pfeifer & Scheier, 1999). Furthermore, the “thinking” component of this cycle
is emphasized far more than either the “sensing” or the “acting.” “One problem with
psychology’s attempt at cognitive theory has been our persistence in thinking about
cognition without bringing in perceptual and motor processes” (Newell, 1990, p. 15).
Embodied cognitive science (Agre, 1997; Brooks, 1999, 2002; Chemero, 2009;
Clancey, 1997; Clark, 1997, 2003, 2008; Pfeifer & Scheier, 1999; Robbins & Aydede, 2009;
Shapiro, 2011; Varela, Thompson, & Rosch, 1991) recognizes the importance of sensing
and acting, and reacts against central cognitive control. Its more radical proponents
strive to completely replace the sense-think-act cycle with sense-act mechanisms.
This reaction is consistent with several themes in the current chapter: the
importance of the environment, degrees of embodiment, feedback between the
world and the agent, and the integral relationship between an agent’s body and its
umwelt. Given these themes, it becomes quite plausible to reject the proposal that
cognition is used to plan, and to posit instead that the purpose of cognition is to
guide action:
The brain should not be seen as primarily a locus of inner descriptions of external
states of affairs; rather, it should be seen as a locus of internal structures that act as
operators upon the world via their role in determining actions. (Clark, 1997, 47)
Importantly, these structures do not stand between sensing and acting, but instead
provide direct links between them.
The action-based reaction against classical cognitivism is typified by pioneering
work in behaviour-based robotics (Brooks, 1989, 1991, 1999, 2002; Brooks &
Flynn, 1989). Roboticist Rodney Brooks construes the classical sandwich as a set
of vertical processing layers that separate perception and action. His alternative
is a hierarchical arrangement of horizontal processing layers that directly connect
perception and action.
Brooks’ action-based approach to behaviour is called the subsumption architecture
(Brooks, 1999). The subsumption architecture is a set of modules. However,
these modules are somewhat different in nature than those that were discussed in
Chapter 3 (see also Fodor, 1983). This is because each module in the subsumption
architecture can be described as a sense-act mechanism. That is, every module can
have access to sensed information, as well as to actuators. This means that modules
in the subsumption architecture do not separate perception from action. Instead,
each module is used to control some action on the basis of sensed information.
The subsumption architecture arranges modules hierarchically. Lower-level
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