My argument in this book is as follows: human thought is intimately connected with the activities of the human brain; other vertebrate animals apart from ourselves have very complicated brains, and in some cases brains which appear to be physically very much like our own; this suggests that what goes on in animal brains has a good deal in common with what goes on in human brains; and laboratory experiments on animal behaviour provide some measure of support for this suggestion. In presenting this argument I deal with philosophical opinion about animal psychology, anatomical and physiological studies of the brain, evolutionary theory and the natural life of vertebrate species, and experimental tests of the psychological capacities of animals. I am very much aware that I have given an incomplete account of all these specialised areas, but I have tried to include enough material to give the intelligent layman, or the intelligent undergraduate, a general impression of what kinds of information are available. Some gaps now seem particularly glaring: I have deliberately avoided the question of how theories of animal psychology might impinge on opinions about our moral responsibilities towards animals, but in evolutionary biology and brain physiology many new findings and hypotheses have simply been missed out.
I conclude that it makes sense to suppose that awareness and mental organisation occur in animals, without the involvement of language, but I do not deny that human speech and writing Constitute an exceptional influence on mental activity. In the following pages I have attempted to minimise the use of technical terms, in the hope that there may be some underlying ideas which are more accessible without them.