Googling Schopenhauer's quote led me to wikipedia and some intresting stuff...
“Man can do what he wills but he cannot will what he wills.”
― Arthur Schopenhauer
[A]s little as a ball on a billiard table can move before receiving an impact, so little can a man get up from his chair before being drawn or driven by a motive. But then his getting up is as necessary and inevitable as the rolling of a ball after the impact. And to expect that anyone will do something to which absolutely no interest impels them is the same as to expect that a piece of wood shall move toward me without being pulled by a string.
— Ibid.
[L]et us imagine a man who, while standing on the street, would say to himself: "It is six o'clock in the evening, the work day is over. Now I can go for a walk, or I can go to the club; I can also climb up the tower to see the sun set; I can go to the theater; I can visit this friend or that one; indeed, I also can run out of the gate, into the wide world, and never return. All of this is strictly up to me, in this I have complete freedom. But still I shall do none of these things now, but with just as free a will I shall go home to my wife."
— Chapter III
[M]an does at all times only what he wills, and yet he does this necessarily. But this is because he already is what he wills.
— Ch. V