Sensory Discrimination 59 



quality entering into our own experience, we cannot say. 

 The light rays which to us are red and blue may for an 

 animal's consciousness also differ from each other, and yet 

 if our experience could be exchanged for the animal's, we 

 might find in the latter nothing like red and blue as we know 

 them. 



Thus much being premised, what sort of evidence can 

 be obtained that an animal does discriminate between two 

 stimuli? Again, as in considering the evidence for the 

 existence of consciousness in general, there is an argument 

 from structure and an argument from behavior. 



14. Structure as Evidence of Discrimination 



The argument from structure consists primarily in the fact 

 that an animal possesses sense organs recognizably like 

 our own. If a creature has an organ suggesting strongly the 

 construction of the human cochlea, or an organ with a lens 

 and a membrane composed of rods and cones, it is highly 

 probable that auditory stimuli in the one case and light in the 

 other produce specific sensations. This argument from the 

 morphology of sense organs. is, however, limited in two ways. 

 First, it is only a small part of the animal world whose sense 

 organs resemble ours closely enough to make the analogy 

 safe. And secondly, we do not after all know very much 

 about the relation of our own sense-organ structure to function. 

 We know, for example, that our own organ with a lens and 

 retina gives us visual sensations, but we cannot say with 

 certainty which structures in the retina furnish brightness 

 sensations and which color sensations, nor do we know any- 

 thing about the retinal structures that underlie different 

 qualities of color sensations. We can say that sensations 

 of hearing come from the ear, but no one can tell us how 

 to judge from the structure of the ear what range and 



