The CPC still considers private ownership to be non-socialist.  However, according to party theorists, the existence and growth of private ownership does not necessarily undermine socialism and promote capitalism in China.  It is argued that Karl Marx and Friedrich Engels —the founders of communism—never proposed the immediate abolishment of private ownership.  According to Engel's book Principles of Communism , the proletariat can only abolish private ownership when the necessary conditions have been met.  In the phase before the abolishment of private ownership, Engels proposed progressive taxation, high inheritance taxes and compulsory bond purchases to restrict private property while using the competitive powers of state-owned enterprises to expand the public sector.  Marx and Engels proposed similar measures in the Communist Manifesto in regards to advanced countries, but since China was economically undeveloped, party theorists called for flexibility regarding the CPC's handling of private property.  According to party theorist Liu Shuiyuan, the New Economic Policy program initiated by Soviet authorities in the aftermath of the war communism program is a good example of flexibility by socialist authorities.