Legal Principle: A person is said to be of sound mind for the purpose of making a contract if - at the time when he makes it - he is incapable of understanding it and of forming a rational judgment as to its eff ect upon his interests. Facts: Mr X who is usually of sound state of mind - but occasionally of unsound state of mind - enters into a contract with Mr Y-when he was of unsound state of mind. Mr Y having come to know about this fact afterwards - wants to file a suit against Mr X.