p.191 To be free is to be able to make an unforced choice; and choice entails competing possibilities - at the very least two 'open', unimpeded alternatives. And this, in turn, may well depend on external circumstances which leave only some paths unblocked. When we speak of the extent of freedom enjoyed by a man or a society, we have in mind, it seems to me, the width or extent of the paths before them, the number of open doors, as it were, and the extent to which they are open.

p.191 Some doors are much more important than others - the goods to which they lead are far more central in an individual's or society's life; some doors lead to other open doors, some to closed ones; there is actual and there is potential freedom - depending on how easily some closed doors can be opened given existing or potential resources, physical or mental. How is one to measure one situation against another?

p.192 if I am ignorant, obsessed, irrational, I am thereby blinded to the facts, and a man so blinded is, in effect, as unfree as a man whose possibilities are objectively blocked... The extent of freedom depends on opportunities of action, not on knowledge of them, although such knowledge may well be an indispensable condition for the use of freedom, and although impediments in the path to it are themselves a deprivation of freedom - of freedom to know. Ignorance blocks paths, and knowledge opens them.

p.193 It is worth noting that it is the actual doors that are open that determine the extent of someone's freedom, and not his own preferences.