In The Sun Also Rises, what does Jake think is his function in life?
As a member of one of the so called "Lost Generation," Jake is a character whose cynicism and disillusionment with life caused by the First World War causes him to lead a life that is characterised by inane activity and debauched drunkenness. He and his friends such as Cohn wander around Europe, drinking lots and leaving havoc in their wake. Whenever they get tired of one place, or begin to perceive a void within them, they simply move to another location and begin all over again. Note how this is highlighted explicitly by Jake in Chapter II when Cohn tries to convince Jake to go to South America with him. Jake replies:
You can’t get away from yourself by moving from one place to another.
Jake recognises that the void that Cohn senses within himself will not be filled by simply moving to a new location. He understands that this is a void that he shares, and that there is no simple cure for it, as it is a problem that is part of Cohn and himself and will be taken along with them wherever they go. Jake's purpose in life can therefore be summarised as spending his days in idle pleasures and hedonistic enjoyment, doing everything he can to not face the realities of his own impotence, which is what separates him from Brett.