Chitty on Contracts, 28th edition at page 838, it is stated : "Objects which on grounds of public policy invalidate contracts may, for convenience, be generally classified into five groups : first objects which are illegal by common law or by legislation: secondly, objects injurious to good government either in the field of domestic or foreign affairs: thirdly, objects which interfere with the proper working of the machinery of justice; fourthly, objects, injurious to marriage and morality; and fifthly, objects economically against the public interest. This classification is adopted primarily for case of exposition. Certain cases do not fit clearly into any of these five categories."