 In Two Concepts of Liberty Berlin sought to explain the difference between two (out of more than two hundred, he said) different ways of thinking about political liberty. These, he said, had run through modern thought, and were central to the ideological struggles of his day. Berlin called these two conceptions of liberty negative and  positive.[23]  Berlin’s treatment of these concepts was less than fully even-handed from the start: while he defined negative liberty fairly clearly and simply, he gave positive liberty two different basic definitions, from which still more distinct conceptions would branch out. Negative liberty Berlin initially defined as freedom from, that is, the absence of constraints on the agent imposed by other people. Positive liberty he defined both as freedom to, that is, the ability (not just the opportunity) to pursue and achieve willed goals; and also as autonomy or self-rule, as opposed to dependence on others. These are not the same.