Hierarchies do not always work. "When two agents need to use each other's skills, then neither one can be `on top'." For example, SEE and MOVE are two agents that need each other -- "MOVE to SEE, SEE to MOVE".
"Most of the diagrams in the early parts of this book depict simple hierarchies. Later, we'll see more cross-connected rings and loops -- when we are forced to consider the need for memory, which will become a constant subject of concern in this book." With this passage, a new theme for the text is introduced -- the need for memory, and how it affects control structures.
"If we have enough memory, we can arrange our agents into circular loops and thus use the same agents over and over again to do parts of several different jobs at the same time." This introduces a second critical theme, the recursive nature of mental operations. Minsky's description reminds me a lot about how recursion is implemented in LOGO and LISP.
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