34 The Animal Mind 



animals as high in the scale as dogs and cats learn to solve 

 problems analogous to that of the combination lock so slowly 

 that we cannot infer the presence of ideas. Are we then to 

 conclude that these animals are unconscious, or that there is 

 absolutely no reason for supposing them possessed of con- 

 sciousness? Yerkes has criticised the "learning by expe- 

 rience" criterion by pointing out that " no organism . . . has 

 thus far been proved incapable of profiting by experience." 

 It is a question rather of the rapidity and of the kind of learn- 

 ing involved. "The fact that the crayfish needs a hundred 

 or more experiences for the learning of a type of reaction that 

 the frog would learn with twenty experiences, the dog with 

 five, say, and the human subject with perhaps a single ex- 

 perience, is indicative of the fundamental difficulty in the use 

 of this sign" (463). Nagel has pointed out that Loeb, in 

 asserting "associative memory" as the criterion of conscious- 

 ness, offers no evidence for his statement (294). The fact is 

 that while proof of the existence of mind can be derived from 

 animal learning by experience only if the learning is very 

 rapid, other evidence, equally valid on the principle of anal- 

 ogy, makes it highly improbable that all animals which learn 

 too slowly to evince the presence of ideas are therefore uncon- 

 scious. This evidence is of a morphological character. 



7. Inferring Mind from Structure 



Both Yerkes and Lukas urge that the resemblance of an 

 animal's nervous system and sense-organs to those of human 

 beings ought to be taken into consideration in deciding whether 

 the animal is conscious or not. Lukas suggests that the cri- 

 teria of consciousness should be grouped under three heads : 

 morphological, including the structure of the brain and sense- 

 organs, physiological, and teleological. Under the second 



