Isaiah Berlin on Freedom

Currently I am reading " Freedom and its Betrayal - Six enemies of Human Liberty." originally six lectures each one hour long broadcast over the BBC Radio's Third Programme they have been rescued and edited by Henry Hardy. As with much of Berlin's work they are remarkable for the clarity of thought and, especially for me, is their applicability to the world today.
In his lectures he discusses the ideas of six thinkers who lived at the time of the French revolution and whose work influenced this cataclysm, and our world since.
In the Introduction he asks the question "Why should an individual obey other individuals? Why should any one individual obey other individuals or groups or bodies of individuals?". Actually this this question or a variation of it, is the root subject of all six of the thinkers who are the subject of Berlin's lectures. He paraphrases the question in this interesting sentence. "But for the purposes political philosophy, as opposed to descriptive political theory or sociology, the central question seems to me to be precisely this one: " Why should anyone obey anyone else?"."
In this sentence he separates sociology from political philosophy and dismisses it by calling it " descriptive political theory". I'll come back to this thought at a later date. As I have now opened this can of worms in my usual windy way I'll post it.