18.36-39 Sukham etc. upto udahrtam.  At its time :  at the time of its practice  (use).  Like poison  (1st) :  Because  it is extremely difficult to give up the attachment for sense-objects cultivated during hundreds of [previous] life-periods. That has been said in the revealed literature as :
 
'[The path of sprituality] is the edge of a razor, painful and difficult to cross over etc.'
 
The serenity of intellect  (or mind) results from serenity in the Self, as there exists nothing else to be aspired for.  The  [Rajasic] happiness springs from the mutual contact between the sense-objects and senses, just as in the case of the eye due to its contact with colur.  That happiness which is due to sleep, indolence and heedlessness,  explained earlier, is of the Tamas (Strand).