I intend to insist that both Keynes and Smith considered the common respect of Economics as one of Social Science. Keynes stated that Economics was one of Moral Science, as Smith pointed out the difference between Moral philosophy and Natural philosophy. However, Hume emphasized Moral philosophy should adopted Newton’s experimental method in the analysis of human nature. Keynes doubted whether human acts were based on Mathematical expectation, and indicated the instability due to the characteristic of human nature. Equally Smith submitted that the casuists made mistakes their end for the rules of justice. Criticizing Casuistry, Keynes and Smith recognized that Economics connoted the insecurity of human nature which created social dynamics.