  Socrates completes his refutation of the thesis that knowledge is perception by bringing a twelfth and final objection, directed against D1 itself rather than its Protagorean or Heracleitean interpretations. This objection says that the mind makes use of a range of concepts which it could not have acquired, and which do not operate, through the senses: e.g., “existence,” “sameness,” “difference.” So there is a part of thought, and hence of knowledge, which has nothing to do with perception. Therefore knowledge is not perception.