Necessity vs. contingencyIn On the Freedom of the Will, Schopenhauer demonstrates the (well known in philosophy) distinction between necessity and contingency. He calls "necessary" what follows from a given sufficient basis (i.e. that what is already certain – if one knows that the sufficient cause is present). On the other hand, one calls "contingent" or "incidental" (with regard to a sufficient basis) that what does not follow from the latter (so e.g. two unconnected events can be contingent to each other: like when a black cat crosses the street and one's job is lost on the same day). As moral freedom means lack of necessity, it would mean a lack of any basis: it "would have to be defined as absolutely contingent", i.e. an absolute fortuity, or chance.
The question about the freedom of will is thus the question whether something depends on another thing (a state, an event), i.e. is in some way determined by it, or does not depend on anything (then we call it a chance). Or, in other words, whether something can be predicted: whether it is certain (given the presence of absence of the sufficient cause) or not. Cf. Luther's argument: for him everything is a necessity because the Creator knows it already.