1. C Blakemore (1988) The Mind Machine, BBC Books, London.
2. GG Gallup, Jr (1975) Towards an operational definition of self-awareness, in Socioecology and Psychology of Primates ( RH Tuttle ed), pp. 309–341, Mouton, The Hague.
3. RA Gardner and B Gardner (1969) Teaching sign language to a chimpanzee, Science 165: 664–672.
4. J Goodall (1971) In the Shadow of Man, Houghton Mifflin, Boston, Mass.
5. DR Griffin (1984) Animal thinking, Am. Sci. 75 (5): 456–464.