Simon (1962) proposed that a hierarchical decomposition is necessary for the construction of any complex assembly. Without stable sub-assemblies, complex systems require too many pieces of information (parts) to be considered simultaneously. Without decomposition, the exclusion (or failure) of any part causes a complete system failure. Overlooking parts in smaller subsystems is less likely (since there are less things to consider) and failures, which may also disable the entire system, are more easily traced by testing the operation of individual subsystems.
Levels in a hierarchy however need to be completely insulated from levels above below them. Such insulation is known as a strong level and is characteristic of the levels in a computer systems hierarchy (e.g., register-transfer level, program level, etc). However, in weak levels there is interaction between levels; a lower level may be said to "show through" in the upper level. In the human cognitive architecture, the level between the cognitive band and the rational band is weak, primarily because rationality is bounded by the limited resources of the human computational architecture. This allows the human architecture to be evident from human behavior; Newell characterized this as "psychology".
Press HOME to return to the Table of Contents.