… is from page 102 of Frank Knight‘s 1956 collection On the History and Method of Economics; specifically, it is from Knight’s 1928 essay “Historical and Theoretical Issues in the Problem of Modern Capitalism”:
The subjection of workers to owners is certainly less extreme now than it was in the precapitalistic age, due to the fact of impersonal competition. However imperfect and occasionally ineffective competition may be, it is categorically different from servitude embodied in the political constitution of a society.