Sensory experience is not the source of human knowledge. It is also not the test of truth. That is to say, sensory experience is not the sine qua non of human knowledge, and it alone is not the test of truth. Rather, sensory experience is merely one condition among others necessary for the acquisition of knowledge and for the test of truth. For a human being to acquire knowledge he must not only sense the world, he must in addition sense it with a highly developed central nervous system which permits him to indicate that sensed world to himself and to others through the use of language.¹ In order to test the truth of his beliefs, he must not only verify them through his own sense experiences, he must in addition socially validate them.²