Four Essays on Liberty is perhaps the quintessential Berlin work: not a sustained, full-length book, but a collection of essays focused on a few specific topics and approaching them from divergent directions that in the end find coherence and unity. As individual works, the parts of Four Essays on Liberty had caused considerable discussion, even controversy, among serious writers and thinkers. Berlin prepared a long and carefully written introduction for Four Essays on Liberty that addressed the points that had been raised. This was only one more stage of a philosophical dialogue that ran throughout Berlin’s life and career.
Two major ideas emerged from Four Essays on Liberty that have had profound and lasting impact on political philosophy. The first idea was the concept of “negative” and “positive” liberty—briefly, how much one can escape control contrasted with how much one can control others. Although this distinction has become commonplace since the publication of Berlin’s essay, he was the first major political thinker systematically and eloquently to articulate the distinction and its importance, and his discussion of this topic has been hailed as one of the major contributions of his career.
The second major idea, which was even more central to Berlin’s entire philosophy, was the view that human beings, whether as individuals or as members of a society, pursue goals that cannot be viewed as forming a unified whole. Goals equally valid for their particular groups often contradict one another. There is no single, universal truth that is valid for every society, in all places, and at any time (the concept of monism). Berlin demonstrated in terms of classical empiricism that monism is not a valid theory to explain human behavior and human history.
Four Essays on Liberty placed these two ideas on the philosophical record, where they have remained essential elements in the continuing development of political science. Their enduring impact has been to underscore the need for tolerance and generosity in debates on political goals and activities and to inject a healthy dose of skepticism about the reality of ultimate, universal truths for which individual human beings and their liberties must be sacrificed. After these two points were clearly stated by Berlin, they have come to form a part of any serious discussion of political philosophy or the rights and liberties of the individual in society.