When all the other workmen in a shop make common cause with a fellow workman over his separate grievance, and go out on strike in his support, they engage in a "concerted activity" for "mutual aid or protection," although the aggrieved workman is the only one of them who has any immediate stake in the outcome. The rest know that by their action each one of them assures himself, in case his turn ever comes, of the support of the one whom they are all then helping; and the solidarity so established is "mutual aid" in the most literal sense, as nobody doubts. So too of those engaging in a "sympathetic strike," or secondary boycott; the immediate quarrel does not itself concern them, but by extending the number of those who will make the enemy of one the enemy of all, the power of each is vastly increased. It is one thing how far a community should allow such power to grow; but, whatever may be the proper place to check it, each separate extension is certainly a step in "mutual aid or protection."