KATHOPNISHAD CHAPTER 4 SHLOK 3

                                                               येन रूपं रसं गन्धं 

                                                         शब्दान् स्पर्शाꣳश्च मैथुनान् ।

                                                              एतेनैव विजानाति 

                                                     किमत्र परिशिष्यते। एतद्वैतत् ॥ ३

                                            YEN  ROOPAM  RASAM  GABDHAM

                                        SHABDAN  SPARSHASCHA  MAITHUNAT |

                                                      YETEN EV  VIJANATI 

                                      KIMATRA  PARISHISHYATE ||ETAD  VAI  TAT 3 

              

                    BY WHICH ONE KNOWS FORM,TASTE, SMELL SOUND ,TOUCH AND

 DUALISTIC JOY (?SEXUAL PLEASURE)-WHAT IS UNKNOWABLE TO THAT (ATMAN) 

                                                            THIS IS TRUELY THAT ||3

   The Lord of death is telling Nachiket as to what is Atman or SELF . This part has been discussed many times in various scriptures. When we teach a child as to what is the function of eye - the answer that we expect is EYE . and that type of questioning for all the sense organs is there -touch , smell, hear etc. But the eyes are just the instruments of transferring the external information to somewhere deeper . The retina in eyes can not see nor does the optic nerve which carries the information to brain The occipital lobe of the brain does not see it either as when some one is unconscious, the eyes are open  and certainly information is carried in the brain. But the unconscious person can not see. One may say it is MIND that sees. We are told that there is something beyond the Mind and even the Intellect that is the ultimate seer This SELF or Atman is the one that KNOWS ALL  and there is nothing beyond that . The word MAITHUN is used to explain the dualistic pleasure or need for 2 things to perceive any sensation. One needs external stimulus and internal mechanism to perceive. There must a rose for me to see rose. There must be external stimulus touching my skin for me to feel the touch etc. . So Maithuna does not necessarily mean sex pleasure. It simply denotes need for 2 things.

   As a foot note one could argue that the mind and intellect are enough for this perception by sense organs and there is no need for bringing in SELF. But when one looks at mind or intellect -both are changing . The mind is sometimes calm and sometimes agitated. Same with the intellect. Different persons have different intellect  and even in same person the intellect changes from time to time . So the SELF or Atman which does not change is the KNOWER of all.