Political theory is a collection of individual theories which stand side by side, each one more or less impervious to the impact of new observations and to the advent of new theories. Political activity is dangerous. Arising inevitably out of men's ability to influence each other, conferring upon them the benefits of joint endeavor, an indispensable source of social boons, it is also capable of doing great harm. It has been suggested that recognition of the dangers inherent in political activity may have held up the progress of scientific inquiry in politics; but however important this factor, it can hardly serve as a full explanation. There is a most striking contrast between the object studied by medical science, the body of man, and the object of political science, the body politic. The notion of a healthy political body leads to pseudo-restorations of which the Germanic "Holy Roman Empire" is a striking instance.