The very success of collective bargaining hinges upon the solidarity and integrity of the union which makes the bargain. A union capable of enforcing an agreement is a necessary antecedent condition to such a contract. With this fact in mind, one can believe that John Mitchell was not unduly sanguine in stating that "the tendency is toward the growth of compulsory membership . . . and the time will doubtless come when this compulsion will be as general and will be considered as little of a grievance as the compulsory attendance of children at school." There are certain industries so well centralized, however, that their coercive power is greater than that of the labor union, and these have maintained a consistent hostility to the closed shop. The question of the closed shop is, indeed, the most stubborn issue confronting the union. The principle involves the employment of only union men in a shop; it means a monopoly of jobs by members of the union. The issue is as old as the unions themselves and as perplexing as human nature. As early as 1806 it was contended for by the Philadelphia cordwainers and by 1850 it had become an established union policy. While wages and hours are now, in the greater industrial fields, the subject of a collective contract, this question of union monopoly is still open, though there has been some progress towards an adjustment. Wherever the trade agreement provides for a closed shop, the union, through its proper committees and officers, assumes at least part of the responsibility of the discipline. The agreement also includes methods for arbitrating differences. The acid test of the union is its capacity to live up to this trade agreement. For the purpose of forcing its policies upon its employers and society the unions have resorted to the strike and picketing, the boycott, and the union label. When violence occurs, it usually is the concomitant of a strike; but violence unaccompanied by a strike is sometimes used as a union weapon.