Montesquieu

Democracy and aris­to­cracy are not by nature free sta­tes. Political free­dom is found only in mode­rate govern­ments. But it is not always found in mode­rate sta­tes. It is found only when power is not abu­sed ; but time­less expe­rience tells us that any man who holds power is incli­ned to abuse it : he conti­nues until he encoun­ters limits. Strange to say, vir­tue itself requi­res limits.

In order to pre­vent abuse of power, things must be contri­ved so that power checks power. It is pos­si­ble to have a cons­ti­tu­tion such that no one can be for­ced to do the things which are not requi­red of him by law, and not to do those which the law per­mits him to do.