An old and rusty joke about Jean-Paul Sartre relates that once a friend phoned him to ask whether he were free on Thursday night: and Sartre spent the next two years considering what his answer ought to be.

The freedom that Sartre spent much of his life pondering was to do with the nature of the human psyche and how far a human being can have free will.

Another 20thcentury figure preoccupied with liberty in a more political sense was Sir Isaiah Berlin— a Classicist at Corpus Christi College Oxford, who later became Professor of Social and Political Theory at that university. He receives a fascinating profile on Wikipedia:  a man whose ideas were expressed in the spoken word rather than the writing  of books. He was inspirational, as a sample of his lecturing may illustrate:

Such issues are first articulated by Aristotle in the Politics,  where one finds already suggested the idea that only in society can a person find the freedom for fulfilment.  Man is a social animal:  he or she can be free on a desert island,  but this is freedom of the negative kind;  positive freedom— human liberty—is dependent on being accompanied by other people and by the necessaries of life that enable the exercise of it.

In short, human beings create societies and build cities with the purpose of setting themselves free.

See also Berlin’s Two Concepts of Liberty:

and an earlier post:
The political animal… or not