Nils Nilsson, Marcel Schoppers, Dan Shapiro and Michael Braverman provided very valuable criticisms and suggestions. Marvin Minsky, Rodney Brooks, Luc Steels, J.R. Anderson and Jerome Feldman provided inspiration. Jan Torreele, Piet Spiessens and Tony Bell proofread the paper. This work was sponsored through contracts with the EEC (COST13) and the Belgian Ministry of Science (Incentive Action in AI). The author is a fellow of the Belgian National Science Foundation.