law: Indian Contract Act, 1872 
  12. What is a sound mind for the
  
  purposes of contracting
  A person is said to be of sound mind for the purposes of making
  
  a contract, if , at the time when he makes it, he is capable of understanding
  
  it and of forming a rational judgment as to its effect upon his interests.
  A person who is usually of unsound mind, but occasionally of
  
  sound mind, may make a contract when he is of sound mind.
  A person who is usually of sound mind, but occasionally of
  
  unsound mind, may not make a contract when he is of unsound mind.
   Illustrations 
  (a) A patient in a lunatic asylum who is at intervals of sound
  
  mind, may contract during those intervals.
  (b) A sane man, who is delirious from fever, or who is so drunk
  
  that he cannot understand the terms of a contract, or form a rational judgment
  
  as to its effect on his interests, cannot contract whilst such delirium of
  
  drunkenness lasts.