Levels, hierarchies, and the locus of control

The Bartlett--Craik view of human performance is restated; and particularly that it is organised at different levels. On that view, the lower levels are controlled by the upper but capable of functioning independently. Modern views of memory, language, and problem-solving are compared with this doctrine, and found to embody some of its virtues but not all. Fresh experiments are described, in which people take decisions about the running of a transportation system. The simplest control mechanism which will model their behaviour is a two-level adaptive controller.